PHP Runescape Level Formula Functions

<?php
function experience($xp) {
  $a=0;
  $count = 0;
  $last = 0;
  for($x=1; $x<128; $x++) {
    $last = floor($a/4);
    $a += floor($x+300*pow(2, ($x/7)));
    $count += 1;
    if (($last >= $xp) && ($last <= floor($a/4))) {
        return $count -= 1;
    }
  }
}
function experience_underscore($xp) {
    $a=0;
    $count = 0;
    $last = 0;
    for($x=1; $x<128; $x++) {
        $last = floor($a/4);
        $a += floor($x+300*pow(2, ($x/7)));
        $count += 1;
        if (($last >= $xp) && ($last <= floor($a/4))) {
            if (($count -= 1) > 99) {
                return ($count);
            } else {
                return $count;
            }
        }
    }
}
function experience_to_level($L) {
    $a=0;
    for($x=1; $x<$L; $x++) {
        $a += floor($x+300*pow(2, ($x/7)));
    }
    return floor($a/4);
}

function experience_to_next_level($exp, $name = '') {
    $nextlevel = experience($xp = $exp) + 1;
    $required = experience_to_level($L = $nextlevel);
    return number_format($required - $exp);
}
?>