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copied!<p>There are three parts to the validation of the card number:</p> <ol> <li><strong>PATTERN</strong> - does it match an issuers pattern (e.g. VISA/Mastercard/etc.)</li> <li><strong>CHECKSUM</strong> - does it actually check-sum (e.g. not just 13 random numbers after "34" to make it an AMEX card number)</li> <li><strong>REALLY EXISTS</strong> - does it actually have an associated account (you are unlikely to get this without a merchant account)</li> </ol> <h2>Pattern</h2> <ul> <li>MASTERCARD Prefix=51-55, Length=16 (Mod10 checksummed)</li> <li>VISA Prefix=4, Length=13 or 16 (Mod10)</li> <li>AMEX Prefix=34 or 37, Length=15 (Mod10)</li> <li>Diners Club/Carte Prefix=300-305, 36 or 38, Length=14 (Mod10)</li> <li>Discover Prefix=6011,622126-622925,644-649,65, Length=16, (Mod10)</li> <li>etc. (<a href="http://en.wikipedia.org/wiki/Bank_card_number#Issuer_identification_number_.28IIN.29" rel="noreferrer">detailed list of prefixes</a>)</li> </ul> <h2>Checksum</h2> <p>Most cards use the Luhn algorithm for checksums:</p> <p><a href="http://en.wikipedia.org/wiki/Luhn_algorithm" rel="noreferrer">Luhn Algorithm described on Wikipedia</a></p> <p>There are links to many implementations on the Wikipedia link, including PHP:</p> <pre><code><? /* Luhn algorithm number checker - (c) 2005-2008 shaman - www.planzero.org * * This code has been released into the public domain, however please * * give credit to the original author where possible. */ 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; $i<$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 > 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; } ?> </code></pre>

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