Luhn 10 algorithm for credit card check

The luhn_check() function is a small PHP function which checks whether a number is a valid based on the Luhn algoritm.
This function may be used to check for valid credit card numbers, however additional checks may be desired, such as
card prefix and length checks.

<pre><code>&lt;?

function luhn_check($number) {

// Strip any non-digits (useful for credit card numbers with spaces and hyphens)
$number=preg_replace(‘/\D/’, ”, $number);

// Set the string length and parity
$number_length=strlen($number);
$parity=$number_length % 2;

// Loop through each digit and do the maths
$total=0;
for ($i=0; $i&lt;$number_length; $i++) {
$digit=$number[$i];
// Multiply alternate digits by two
if ($i % 2 == $parity) {
$digit*=2;
// If the sum is two digits, add them together (in effect)
if ($digit &gt; 9) {
$digit-=9;
}
}
// Total up the digits
$total+=$digit;
}

// If the total mod 10 equals 0, the number is valid
return ($total % 10 == 0) ? TRUE : FALSE;

}

?&gt;</code></pre>