If you are looking for a code function to generate a random expression, like password with alphanumeric or any other generate a random expression, use this function:

<?php
function GeraHash($qtd){
//Under the string $Caracteres you write all the characters you want to be used to randomly generate the code.
$Caracteres = 'ABCDEFGHIJKLMOPQRSTUVXWYZ0123456789';
$QuantidadeCaracteres = strlen($Caracteres);
$QuantidadeCaracteres--;

$Hash=NULL;
for($x=1;$x<=$qtd;$x++){
$Posicao = rand(0,$QuantidadeCaracteres);
$Hash .= substr($Caracteres,$Posicao,1);
}

return $Hash;
}

//Here you specify how many characters the returning string must have
echo GeraHash(30);
?>

source: http://php.net