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PHP type comparison tables

The following tables demonstrate behaviors of PHP types and comparison operators, for both loose and strict comparisons. This supplemental is also related to the manual section on type juggling. Inspiration was provided by various user comments and by the work over at » BlueShoes.

Before utilizing these tables, it's important to understand types and their meanings. For example, "42" is a string while 42 is an integer. FALSE is a boolean while "false" is a string.

Note:
HTML Forms do not pass integers, floats, or booleans; they pass strings. To find out if a string is numeric, you may use is_numeric().

Note:
Simply doing if ($x) while $x is undefined will generate an error of level E_NOTICE. Instead, consider using empty() or isset() and/or initialize your variables.

Note:
Some numeric operations can result in a value represented by the constant NAN. Any loose or strict comparisons of this value against any other value, including itself, but except TRUE, will have a result of FALSE. (i.e. NAN != NAN and NAN !== NAN) Examples of operations that produce NAN include sqrt(-1), asin(2), and acosh(0).

**Comparisons of `$x` with PHP functions**
Expression	gettype()	empty()	is_null()	isset()	boolean : if($x)
$x = "";	string	`TRUE`	`FALSE`	`TRUE`	`FALSE`
$x = null;	NULL	`TRUE`	`TRUE`	`FALSE`	`FALSE`
var $x;	NULL	`TRUE`	`TRUE`	`FALSE`	`FALSE`
`$x` is undefined	NULL	`TRUE`	`TRUE`	`FALSE`	`FALSE`
$x = array();	array	`TRUE`	`FALSE`	`TRUE`	`FALSE`
$x = array('a', 'b');	array	`FALSE`	`FALSE`	`TRUE`	`TRUE`
$x = false;	boolean	`TRUE`	`FALSE`	`TRUE`	`FALSE`
$x = true;	boolean	`FALSE`	`FALSE`	`TRUE`	`TRUE`
$x = 1;	integer	`FALSE`	`FALSE`	`TRUE`	`TRUE`
$x = 42;	integer	`FALSE`	`FALSE`	`TRUE`	`TRUE`
$x = 0;	integer	`TRUE`	`FALSE`	`TRUE`	`FALSE`
$x = -1;	integer	`FALSE`	`FALSE`	`TRUE`	`TRUE`
$x = "1";	string	`FALSE`	`FALSE`	`TRUE`	`TRUE`
$x = "0";	string	`TRUE`	`FALSE`	`TRUE`	`FALSE`
$x = "-1";	string	`FALSE`	`FALSE`	`TRUE`	`TRUE`
$x = "php";	string	`FALSE`	`FALSE`	`TRUE`	`TRUE`
$x = "true";	string	`FALSE`	`FALSE`	`TRUE`	`TRUE`
$x = "false";	string	`FALSE`	`FALSE`	`TRUE`	`TRUE`

**Loose comparisons with ==**
	`TRUE`	`FALSE`	1	0	-1	"1"	"0"	"-1"	`NULL`	array()	"php"	""
`TRUE`	`TRUE`	`FALSE`	`TRUE`	`FALSE`	`TRUE`	`TRUE`	`FALSE`	`TRUE`	`FALSE`	`FALSE`	`TRUE`	`FALSE`
`FALSE`	`FALSE`	`TRUE`	`FALSE`	`TRUE`	`FALSE`	`FALSE`	`TRUE`	`FALSE`	`TRUE`	`TRUE`	`FALSE`	`TRUE`
1	`TRUE`	`FALSE`	`TRUE`	`FALSE`	`FALSE`	`TRUE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`
0	`FALSE`	`TRUE`	`FALSE`	`TRUE`	`FALSE`	`FALSE`	`TRUE`	`FALSE`	`TRUE`	`FALSE`	`TRUE`	`TRUE`
-1	`TRUE`	`FALSE`	`FALSE`	`FALSE`	`TRUE`	`FALSE`	`FALSE`	`TRUE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`
"1"	`TRUE`	`FALSE`	`TRUE`	`FALSE`	`FALSE`	`TRUE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`
"0"	`FALSE`	`TRUE`	`FALSE`	`TRUE`	`FALSE`	`FALSE`	`TRUE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`
"-1"	`TRUE`	`FALSE`	`FALSE`	`FALSE`	`TRUE`	`FALSE`	`FALSE`	`TRUE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`
`NULL`	`FALSE`	`TRUE`	`FALSE`	`TRUE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`TRUE`	`TRUE`	`FALSE`	`TRUE`
array()	`FALSE`	`TRUE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`TRUE`	`TRUE`	`FALSE`	`FALSE`
"php"	`TRUE`	`FALSE`	`FALSE`	`TRUE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`TRUE`	`FALSE`
""	`FALSE`	`TRUE`	`FALSE`	`TRUE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`TRUE`	`FALSE`	`FALSE`	`TRUE`

Strict comparisons with *===*
	`TRUE`	`FALSE`	1	0	-1	"1"	"0"	"-1"	`NULL`	array()	"php"	""
`TRUE`	`TRUE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`
`FALSE`	`FALSE`	`TRUE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`
1	`FALSE`	`FALSE`	`TRUE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`
0	`FALSE`	`FALSE`	`FALSE`	`TRUE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`
-1	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`TRUE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`
"1"	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`TRUE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`
"0"	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`TRUE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`
"-1"	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`TRUE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`
`NULL`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`TRUE`	`FALSE`	`FALSE`	`FALSE`
array()	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`TRUE`	`FALSE`	`FALSE`
"php"	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`TRUE`	`FALSE`
""	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`FALSE`	`TRUE`

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User Contributed Notes 21 notes

down

php at richardneill dot org ¶

7 years ago


[Editor's note: As of PHP 5.4.4 this is no longer true. Integral strings that overflow into floating point numbers will no longer be considered equal.]



Be wary of string-comparison where both strings might be interpreted as numbers.  Eg:



$x="123456789012345678901234567890"; $y="123456789012345678900000000000";

echo  ($x==$y)?"equal":"not_equal";   #Prints equal !!



Both strings are getting converted to floats, then losing precision, then becoming equal :-(



Using "===" or making either of the strings non-numeric will prevent this.

[This is on a 32-bit machine, on a 64-bit, you will have to make the strings longer to see the effect]

down

php at benizi dot com ¶

9 years ago


It's interesting to note that 'empty()' and 'boolean : if($x)'
are paired as logical opposites, as are 'is_null()' and 'isset()'.

down

frank ¶

12 years ago


A comparison table for <=,<,=>,> would be nice...
Following are TRUE (tested PHP4&5):
NULL <= -1
NULL <= 0
NULL <= 1
!(NULL >= -1)
NULL >= 0
!(NULL >= 1)
That was a surprise for me (and it is not like SQL, I would like to have the option to have SQL semantics with NULL...).

down

aidan at php dot net ¶

14 years ago


The way PHP handles comparisons when multiple types are concerned is quite confusing.

For example:
"php" == 0

This is true, because the string is casted interally to an integer. Any string (that does not start with a number), when casted to an integer, will be 0.

down

Jan ¶

13 years ago


Note that php comparison is not transitive:

"php" == 0 => true
0 == null => true
null == "php" => false

down

alt dot jl-3gi8b7l at binich dot com ¶

1 year ago


Here's how PHP handles loose and strict comparisons of default parameters in functions:

Very unintuitive:

<?php
function f($x='surprise')
    {
    if ($x == 'surprise')
        return $x; // returns 0 !!
    }
    
echo f(0);
?>

Here some further tests:

<?php
function f($x='surprise')
    {
    if ($x == 'surprise')
        return 'A:'.$x;
    if ($x === 'surprise')
        return 'B:'.$x;
    return 'C:'.$x;
    }
    
echo "\n".f(0); // A:0 !!
echo "\n".f(0.0); // A:0 !!
echo "\n".f(NULL); // C: !!
echo "\n".f(FALSE); // C:
echo "\n".f(); // A:surprise
echo "\n".f(''); // C:
echo "\n".f((integer)0); // A:0
echo "\n".f((string)0); // C:0
echo "\n".f('0'); // C:0
echo "\n".f(0.1); // C:0.1
echo "\n".f(array()); // C:Array
echo "\n".f('surprise'); // A:surprise
?>

down

jerryschwartz at comfortable dot com ¶

14 years ago


In some languages, a boolean is promoted to an integer (with a value of 1 or -1, typically) if used in an expression with an integer. I found that PHP has it both ways:

If you add a boolean with a value of true to an integer with a value of 3, the result will be 4 (because the boolean is cast as an integer).

On the other hand, if you test a boolean with a value of true for equality with an integer with a value of three, the result will be true (because the integer is cast as a boolean).

Surprisingly, at first glance, if you use either < or > as the comparison operator the result is always false (again, because the integer as cast as a boolean, and true is neither greater nor less than true).

down

edgar at goodforall dot eu ¶

9 years ago


Some function to write out your own comparisson table in tsv format. Can be easily modified to add more testcases and/or binary functions. It will test all comparables against each other with all functions. 

<?php
$funcs = array(
        /* Testing equality */
        'eq' => '==',
        'ne' => '!=',
        'gt' => '>',
        'lt' => '<',
        'ne2' => '<>',
        'lte' => '<=',
        'gte' => '>=',
        /* Testing identity */
        'id' => '===',
        'nid' => '!=='
);
class Test {
        protected $a;
        public $b;
        public function __construct($a,$b){
                $this->a = $a;
                $this->b = $b;
        }
        public function getab(){
                return $this->a.",". $this->b;
        }

}
$tst1 = new Test(1,2);
$tst2 = new Test(1,2);
$tst3 = new Test(2,2);
$tst4 = new Test(1,1);

$arr1 = array(1,2,3);
$arr2 = array(2,3,4);
$arr3 = array('a','b','c','d');
$arr4 = array('a','b','c');
$arr5 = array();

$comp1 = array(
        'ints' => array(-1,0,1,2),
        'floats' => array(-1.1,0.0,1.1,2.0),
        'string' => array('str', 'str1', '', '1'),
        'bools' => array(true, false),
        'null' => array(null),
        'objects' => array($tst1,$tst2,$tst3,$tst4),
        'arrays' => array($arr1, $arr2, $arr3, $arr4, $arr5)
);
$fbody = array();

foreach($funcs as $name => $op){
        $fbody[$name] = create_function('$a,$b', 'return $a ' . $op . ' $b;');
}

$table = array(array('function', 'comp1', 'comp2', 'f comp1 comp2', 'type'));
/* Do comparisons */
$comp2  = array();
foreach($comp1 as $type => $val){
        $comp2[$type] = $val;
}

foreach($comp1 as $key1 => $val1){
        foreach($comp2 as $key2 => $val2){
                addTableEntry($key1, $key2, $val1, $val2);
        }
}
$out = '';
foreach($table as $row){
        $out .= sprintf("%-20s\t%-20s\t%-20s\t%-20s\t%-20s\n", $row[0], $row[1], $row[2], $row[3], $row[4]);
}

print $out;
exit;

function addTableEntry($n1, $n2, $comp1, $comp2){
        global $table, $fbody;
        foreach($fbody as $fname => $func){
                        foreach($comp1 as $val1){
  foreach($comp2 as $val2){
                                        $val = $func($val1,$val2);
                                                $table[] = array($fname, gettype($val1) . ' => ' . sprintval($val1), gettype($val2) .' => ' . sprintval($val2), gettype($val) . ' => ' . sprintval($val), gettype($val1) . "-" . gettype($val2) . '-' . $fname);
                                        }
                        }
        }
}

function sprintval($val){
        if(is_object($val)){
                return 'object-' . $val->getab();
        }
        if(is_array($val)){
                return implode(',', $val);
        }
        if(is_bool($val)){
                if($val){
                        return 'true';
                }
                return 'false';
        }
        return strval($val);
}

?>

down

blue dot hirano at gmail dot com ¶

5 years ago


The truth tables really ought to be colorized; they're very hard to read as they are right now (just big arrays of TRUE and FALSE).

Also, something to consider: clustering the values which compare similarly (like is done on qntm.org/equality) would make the table easier to read as well. (This can be done simply by hand by rearranging the order of headings to bring related values closer together).

down

mark at theanti dot social ¶

1 year ago


There is also 0.0 which is not identical to 0.

  $x = 0.0; 
  gettype($x); // double
  empty($x); // true
  is_null($x); //false
  isset($x); // true
  is_numeric($x); // true
  $x ? true : false; // false
  $x == 0; // true
  $x == "0"; // true
  $x == "0.0"; // true
  $x == false; // true
  $x == null; // true
  $x === 0; // false
  $x === false; // false
  $x === null; // false
  $x === "0"; // false
  $x === "0.0"; // false

down

biziclop at vipmail dot hu ¶

2 years ago


Want to strictly compare integers that may or may not be converted to floats (via PHPExcel import/export, etc.)? You are out of luck:

var_dump(   (int)1 ===   (int)1 ); // bool(true)
var_dump( (float)1 === (float)1 ); // bool(true)
var_dump( (float)1 ===   (int)1 ); // bool(false)
var_dump(   (int)1 === (float)1 ); // bool(false)

This slightly complicated function might help you:

function equal_numbers( $a, $b ){
  if( $a === $b )  return true;
  if(   is_int( $a ) && is_float( $b ))  return (float)$a === $b && $a ===   (int)$b;
  if( is_float( $a ) &&   is_int( $b ))  return   (int)$a === $b && $a === (float)$b;
  return false;
}

Here are some tests:

test(100, 100.0 ); // int(100) float(100) bool(true) 
test(  0,  -0   ); // int(0)   int(0)     bool(true) 
test( -0,  -0.0 ); // int(0)   float(-0)  bool(true)
test(  0, 1/0   ); // int(0)   float(INF) bool(false)
test(  0, acos(2));// int(0)   float(NAN) bool(false)
test(       PHP_INT_MAX * 10, 0 );             //float(92233720368547758080) int(0) bool(false)
test((float)PHP_INT_MAX,     PHP_INT_MAX     );//float(9223372036854775808)  int(9223372036854775807) bool(false)
test((float)PHP_INT_MAX / 2, PHP_INT_MAX / 2 );//float(4611686018427387904)  float(4611686018427387904) bool(true)

function test( $a, $b ){
  var_dump( $a, $b, equal_numbers( $a, $b ));
}

down

-1

trexx68 ¶

4 years ago


If you want to view the truth tables colorized, just:

1. Save a local copy of this page as an .html file,
2. View the page source with any text editor.
3. Replace the opening <head> tag so it will include this: 

<head> 
  <script src="https://code.jquery.com/jquery-1.10.2.js"></script>

4. Replace the closing </body> tag with this: 

  <script>
    $( "td:contains('FALSE')" ).css("color", "red");
    $( "td:contains('TRUE')" ).css("color", "green");
  </script>
</body>

5. Save the file, and view it on your browser.  Enjoy.

down

-2

Anonymous ¶

4 years ago


The loose comparison chart is missing a few things.

array(1)==true returns true.

also:

(int)array(1) === 1 returns true

This is symmetric:

array(1) === (array)1 returns true

but the loose comparison

array(1)==1 still returns false.

down

-2

info at shaelf dot ru ¶

11 years ago


Compare object
<?php
$o = new stdClass();
$o->we = 12;

$o2 = new stdClass();
$o2->we = 12;

$o3 = clone $o2;
var_dump($o == $o2); //true
var_dump($o === $o2); //false
var_dump($o3 === $o2); //true
?>

down

-3

Anonymous ¶

5 years ago


I'm running PHP 5.5.3.

This is a correction to one of the previous posts.

<?php
$o = new stdClass();
$o->we = 12;

$o2 = new stdClass();
$o2->we = 12;

$o3 = clone $o2;
var_dump($o == $o2); //true
var_dump($o === $o2); //false
var_dump($o3 === $o2); //false
?>

Output is:

true, false, false

down

-6

crazy888s at hotmail dot com ¶

6 years ago


PHP's loose comparisons can be a huge convenience when used properly! It's extremely helpful to just remember the following are always FALSE:

null, false, "", 0, "0", array()

If your application never depends on a particular "empty/false/null/0/not set" value type, you won't have to worry about 99% of the other weird cases listed here. You won't need empty() or isset(). And ALL variable types will always work as expected for statements like:

if(boolean && !string){...}
if(array){...}
if(!null || int){...}

Consider the same when working with your database values.

down

-4

aravind dot a dot padmanabhan at gmail dot com ¶

4 years ago


please note that
 
$x = 0.0 ;
print empty($x); // returns 1 (true) 

$x = "0.0";
print empty($x); // returns blank (false);

down

-7

engineer dot morozov at gmail dot com ¶

3 years ago


If you want to calculate how much more/less items by the criterion

Example:

in loop:
$counter += intval($item->isValid()) <=> 0.5;

if $counter >= 0 - valid items are more then invalid on $counter
if $counter  < 0 - valid items are less then invalid on $counter

down

-13

Jouriy LYSENKO ¶

8 years ago


If $var not declared.

In php 5.2 :
<?php if($var) ?> - work

in php 5.3 :
<?php if($var) ?> - dont work and generate error E_NOTICE

down

-2

php at ckls dot net ¶

1 year ago


regarding the editors note in php at richardneill dot org note:
[Editor's note: As of PHP 5.4.4 this is no longer true. Integral strings that overflow into floating point numbers will no longer be considered equal.]

While this is true, there is still the following behaviour:
<?php
$x = "1";
$y = "+1";
var_dump($x == $x);
?>
outputs "bool(true)".  
So still be wary of string-comparison where both strings might be interpreted as numbers.

down

-17

tom ¶

14 years ago


<?php

if (strlen($_POST['var']) > 0) {

    // form value is ok

}

?>



When working with HTML forms this a good way to:



(A) let "0" post values through like select or radio values that correspond to array keys or checkbox booleans that would return FALSE with empty(), and;

(B) screen out $x = "" values, that would return TRUE with isset()!



Because HTML forms post values as strings, this is a good way to test variables!



[[Editor Note: This will create a PHP Error of level E_NOTICE if the checked variable (in this case $_POST['var']) is undefined. It may be used after (in conjuection with) isset() to prevent this.]]

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