(PHP 4, PHP 5, PHP 7, PHP 8)
  elseif, as its name suggests, is a combination
  of if and else.  Like
  else, it extends an if
  statement to execute a different statement in case the original
  if expression evaluates to
  false.  However, unlike
  else, it will execute that alternative
  expression only if the elseif conditional
  expression evaluates to true.  For example, the
  following code would display a is bigger than
  b, a equal to b
  or a is smaller than b:
  
<?php
if ($a > $b) {
    echo "a is bigger than b";
} elseif ($a == $b) {
    echo "a is equal to b";
} else {
    echo "a is smaller than b";
}
?>
  There may be several elseifs within the same
  if statement.  The first
  elseif expression (if any) that evaluates to
  true would be executed.  In PHP, it's possible to write
  else if (in two words) and the behavior would be identical
  to the one of elseif (in a single word).  The syntactic meaning
  is slightly different (the same behavior as C) but the bottom line
  is that both would result in exactly the same behavior.
 
  The elseif statement is only executed if the
  preceding if expression and any preceding
  elseif expressions evaluated to
  false, and the current
  elseif expression evaluated to
  true.
 
Note: Note that
elseifandelse ifwill only be considered exactly the same when using curly brackets as in the above example. When using a colon to defineif/elseifconditions, the use ofelseifin a single word becomes necessary. PHP will fail with a parse error ifelse ifis split into two words.
<?php
/* Incorrect Method: */
if ($a > $b):
    echo $a." is greater than ".$b;
else if ($a == $b): // Will not compile.
    echo "The above line causes a parse error.";
endif;<?php
/* Correct Method: */
if ($a > $b):
    echo $a." is greater than ".$b;
elseif ($a == $b): // Note the combination of the words.
    echo $a." equals ".$b;
else:
    echo $a." is neither greater than or equal to ".$b;
endif;
?>