(PHP 4 >= 4.3.0, PHP 5)
sha1 — Calculate the sha1 hash of a string
$str
   [, bool $raw_output = false
  ] )
   Calculates the sha1 hash of str using the
   » US Secure Hash Algorithm 1.
  
strThe input string.
raw_output
       If the optional raw_output is set to TRUE,
       then the sha1 digest is instead returned in raw binary format with a
       length of 20, otherwise the returned value is a 40-character
       hexadecimal number.
      
Returns the sha1 hash as a string.
Example #1 A sha1() example
<?php
$str = 'apple';
if (sha1($str) === 'd0be2dc421be4fcd0172e5afceea3970e2f3d940') {
    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.