Create Simple Code Generator To Reduce Spam With PHP

this is just simple code that i use to deffense spam, i don't know it is work or not, why i use this code. Because before i use it, i try to use the image security that generated also with PHP but it doesn't work :( . So I try the other way, perhapas it can work

<?php

function code($length){

srand((double)microtime()*1000000);

$vowels = array("a", "e", "i", "o", "u");
$cons = array("b", "c", "d", "g", "h", "j", "k", "l", "m", "n", "p", "r", "s", "t", "u", "v", "w", "tr",
"cr", "br", "fr", "th", "dr", "ch", "ph", "wr", "st", "sp", "sw", "pr", "sl", "cl");

$num_vowels = count($vowels);
$num_cons = count($cons);

for($i = 0; $i < $length; $i++){
$password .= $cons[rand(0, $num_cons - 1)] . $vowels[rand(0, $num_vowels - 1)];
}

return substr($password, 0, $length);
}

$si=code(4);

?>

please give me some replay to get the best method to against the spam