(PHP 4, PHP 5)
md5 — Calculate the md5 hash of a string
$str
   [, bool $raw_output = false
  ] )
   Calculates the MD5 hash of str using the
   » RSA Data Security, Inc.
   MD5 Message-Digest Algorithm, and returns that hash.
  
strThe string.
raw_output
       If the optional raw_output is set to TRUE,
       then the md5 digest is instead returned in raw binary format with a
       length of 16.
      
Returns the hash as a 32-character hexadecimal number.
Example #1 A md5() example
<?php
$str = 'apple';
if (md5($str) === '1f3870be274f6c49b3e31a0c6728957f') {
    echo "Would you like a green or red apple?";
}
?>
Note: Secure password hashing
It is not recommended to use this function to secure passwords, due to the fast nature of this hashing algorithm. See here for details.