DEA Number validation function https://en.wikipedia.org/wiki/DEA_number

dea_validate.php

<?php

if (!function_exists('dea_validate')) {

    /**
     * Validate a provided DEA number is valid.
     * 
     * @param  string $deaNumber
     * @return boolean
     */
    function dea_validate($deaNumber)
    {   
        // Make sure we have the correct format.
        if (preg_match('/^[A-Z9]{2}\d{7}$/', $deaNumber) !== 1) {
            return false;
        }

        // Grab all characters in the number apart from the
        // first one as we don't need it for calc.
        $characters = array_slice(str_split((string)$deaNumber), 2);

        // Grab the last digit as it's what we check.
        $checkDigit = $characters[count($characters) - 1];

        // Our counters
        $even = 0;
        $odd = 0;

        // Go through and sum up our digits.
        foreach ($characters as $i => $character) {
            if ($i > 5) {
                break;
            }

            $digit = (int)$character;

            if ($i % 2 == 0) {
                $even += $digit;
            } else {
                $odd += $digit;
            }
        } 

        // Double the odd
        $odd *= 2;
        $sum = $even + $odd;

        // Grab the check number from the sum.
        $checksum = current(array_slice(str_split((string)$sum), 1));

        // The check digit needs to match our checksum.
        return (int)$checkDigit === (int)$checksum;
    }
}