cc_expire_validators.php

<?php
function valid_cc_expire_format($val) {
	return (bool) preg_match('/\d{2}\/\d{4}/', $val);
}

function valid_cc_expire_value($val) {
	$currentMonth = intval(date('n'));
	$currentYear = intval(date('Y'));
	$r = preg_match('/(\d{2})\/(\d{4})/', $val, $matches);

	// Could not match valid input
	if (!$r) {
		return false;
	}

	// Pull out the given month/year values for easier use
	$givenMonth = intval($matches[1]);
	$givenYear = intval($matches[2]);

	// Make sure we are in a valid month/year range
	// A valid year is defined as the current year or higher
	// Be pessimistic and assume the card is expired by default
	$isValidYear = $givenYear >= $currentYear;
	$isValidMonth = false;

	// Month-checking is a little more nuanced
	// We can't have a past month in the current year
	// but we _can_ have a month in a future year
	if ($isValidYear) {
		// The card expires this year, make sure the month hasn't passed
		if ($givenYear === $currentYear) {
			$isValidMonth = $givenMonth >= $currentMonth;

			// If it's not the current year, then the card expires in the future
			// In that case, all months are valid
		} else {
			$isValidMonth = true;
		}
	}

	return $isValidMonth && $isValidYear;
}

function check_valid_cc_expire($val) {
	return valid_cc_expire_format($val) && valid_cc_expire_value($val);
}