mcrypt_generic_init

int mcrypt_generic_init ( resource $td , string $key , string $iv )

This function initializes all buffers needed for encryption

You need to call this function before every call to mcrypt_generic() or mdecrypt_generic().

If you write error-checking into your code, be warned that this function returns FALSE on some errors, and 0 on success, hence
Following is the example

 
mcrypt_generic_init( $a, $b, $c ) or die( "Oops"); 
ALWAYS exits with the error message "Oops", and 
(mcrypt_generic_init( $a, $b, $c ) >= 0 ) or die( "Oops"); 
Sometimes continues when mcrypt_generic_init() actually failed. 
To check for successful init use something like: 
$s = mcrypt_generic_init( $a, $b, $c ); 
if( ($s < 0) || ($s === false)) 
die( "error" );