The readline library is not available on Windows.
<?php
if (PHP_OS == 'WINNT') {
  echo '$ ';
  $line = stream_get_line(STDIN, 1024, PHP_EOL);
} else {
  $line = readline('$ ');
}
?>
(PHP 4, PHP 5, PHP 7)
readline — Reads a line
$prompt = NULL
  ] ) : stringReads a single line from the user. You must add this line to the history yourself using readline_add_history().
promptYou may specify a string with which to prompt the user.
   Returns a single string from the user. The line returned has the ending
   newline removed.
   If there is no more data to read, then FALSE is returned.
  
Example #1 readline() Example
<?php
//get 3 commands from user
for ($i=0; $i < 3; $i++) {
        $line = readline("Command: ");
        readline_add_history($line);
}
//dump history
print_r(readline_list_history());
//dump variables
print_r(readline_info());
?>
The readline library is not available on Windows.
<?php
if (PHP_OS == 'WINNT') {
  echo '$ ';
  $line = stream_get_line(STDIN, 1024, PHP_EOL);
} else {
  $line = readline('$ ');
}
?>
If your CLI script accepts input from STDIN and you also want it to prompt for a password (e.g. as mysql client does), then readline() won't work for you. 
What you need to do is read from the terminal device as shown below.
function readline_terminal($prompt = '') {
    $prompt && print $prompt;
    $terminal_device = '/dev/tty';
    $h = fopen($terminal_device, 'r');
    if ($h === false) {
        #throw new RuntimeException("Failed to open terminal device $terminal_device");
        return false; # probably not running in a terminal.
    }
    $line = rtrim(fgets($h),"\r\n");
    fclose($h);
    return $line;
}
$pass = readline_terminal('Password: ');
Note that readline() will return boolean "false" when the user presses CTRL+D.
Christian's code works well, but if you want to be able to hide the user input and not echo it to screen, you need to add -s to the read command. The code below is an expanded function that allows an optional prompt and optional hiding of the input:
function read_password($prompt=null, $hide=false)
{
    if($prompt) print $prompt;
    $s = ($hide) ? '-s' : '';
    $f=popen("read $s; echo \$REPLY","r");
    $input=fgets($f,100);
    pclose($f);
    if($hide) print "\n";
    return $input;
}
A workaround if readline is not compiled into php, because for example the command is only needed within an installation routine. It works as follows under Linux:
$f=popen("read; echo \$REPLY","r");
$input=fgets($f,100);
pclose($f);        
echo "Entered: $input\n";
To haukew at gmail dot com: 
readline provides more features than reading a single line of input ... your example misses line editing and history. If you don't need that, use something as simple as this:
function readline( $prompt = '' )
{
    echo $prompt;
    return rtrim( fgets( STDIN ), "\n" );
}
If you want to prefill the prompt with something when using readline, this worked for me:
<?php
  function readline_callback($ret)
  {
    global $prompt_answer, $prompt_finished;
    $prompt_answer = $ret;
    $prompt_finished = TRUE;
    readline_callback_handler_remove();
  }
  readline_callback_handler_install('Enter some text> ',
                                    'readline_callback');
  $prefill = 'foobar';
  for ($i = 0; $i < strlen($prefill); $i++)
  {
    readline_info('pending_input', substr($prefill, $i, 1));
    readline_callback_read_char();
  }
  $prompt_finished = FALSE;
  $prompt_answer = FALSE;
  while (!$prompt_finished)
    readline_callback_read_char();
  echo 'You wrote: ' . $prompt_answer . "\n";
?>
In CGI mode be sure to call:
ob_implicit_flush(true);
at the top of your script if you want to be able to output data before and after the prompt.
-- Tomas V.V.Cox
<?php
class ConsoleQuestion
{
    function readline()
    {
        return rtrim(fgets(STDIN));
    }
}
//Example1
$line = new ConsoleQuestion();
$prompt = "What Is Your Name: ";
echo $prompt;
$answer = $line->readline();
echo "You Entered: " . $answer;
//Example2 (comment Ex1 then uncomment Ex2)
/*$prompt = "What Is Your Name: ";
echo $prompt;
$answer =  "You Entered: " . rtrim( fgets( STDIN ));
echo $answer;*/
?>
a few observations....
I use Cygwin PHP v7 and readline is available. The readline_list_history() function though is not defined.
A prompt with escape sequences are sanitized, so use something like:
<?php
    echo("\e[0m\e[34mPromt>\e[0m");
    $inp = readline(' ');
?>
I have not fully documented it, but I see that sometimes strings beginning with punctuation characters do not make it into the history with readline_add_history(). They also sometimes clear the prompt string.
I wanted a function that would timeout if readline was waiting too long... this works on php CLI on linux:
<?php
function readline_timeout($sec, $def)
{
    return trim(shell_exec('bash -c ' .
        escapeshellarg('phprlto=' .
            escapeshellarg($def) . ';' .
            'read -t ' . ((int)$sec) . ' phprlto;' .
            'echo "$phprlto"')));
}
?>
Just call readline_timeout(5, 'whatever') to either read something from stdin, or timeout in 5 seconds and default to 'whatever'.  I tried just using shell_exec without relying on bash -c, but that didn't work for me, so I had to go the round about way.