evolutive script with php - php

Question

I have been trying to improve this script in PHP so it can give me the value of a to 9999999999.....9999999999 (up to 72 characters) to insert in MySQL. So far it stops at 999. I have increased Apache's memory and the script exuction time but it still stays the same. Here is my script:
<?php
function evol($length = 1, $deb_chaine = '') {
$tab=array("a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z","0","1","2","3","4","5","6","7","8","9");
$str = '';
if(strlen($deb_chaine) <= ($length - 1)) {
foreach($tab as $lettre) {
$str .= ' '. $deb_chaine . $lettre;
}
if($deb_chaine == '') {
$str .= evol($length, 'a');
}
else { // sinon
$last = substr($deb_chaine, -1);
$reste = substr($deb_chaine, 0, -1);
if($last == "9") {
$i = strlen($deb_chaine) - 1;
$reste = "";
while($i >= 0) {
if($deb_chaine[$i] == "9") {
$reste = 'a'. $reste;
}
else {
$reste = $tab[(array_search($deb_chaine[$i], $tab) + 1)] . substr($reste, 0, -1);
break 1;
}
$i--;
}
$new = 'a';
}
else {
$new = $tab[(array_search($last, $tab) + 1)];
}
$str .= evol($length, ($reste . $new));
}
}
return $str;
}
echo evol(72);
?>
This code sets the value of a to 999.