Validate Bulgarian ID number (EGN)

This function will allow you to verify if a given string matches the standard for unique ID of Bulgarian citizen (EGN).

/**
* Check if the given parameter is valid ID of Bulgarian citizen (EGN).
* @author Samui Banti - https://samiwell.eu
* @param string $idNumber - A unique ID of Bulgarian citizen.
* @return boolian - TRUE if the ID is valid.
*/
function validateBulgarianID($idNumber)
{
    $weights = [2,4,8,5,10,9,7,3,6];
    
    if ( strlen($idNumber) != 10 ) {
        return false;
    }
        
    $year   = substr($idNumber,0,2);
    $month  = substr($idNumber,2,2);
    $day    = substr($idNumber,4,2);
    
    if ($month > 40) {
        if ( !checkdate($month-40, $day, $year+2000) ) {
            return false;
        }
    } else {
        if ($month > 20) {
            if ( !checkdate($month-20, $day, $year+1800) ) {
                return false;
            }
        } else {
            if ( !checkdate($month, $day, $year+1900) ) {
                return false;
            }
        }
    }
    
    $checksum = substr($idNumber,9,1);
    $idNumberSum = 0;
    for ( $i=0;$i<9;$i++ ) {
        $idNumberSum += substr($idNumber,$i,1) * $weights[$i];
    }
    $valid_checksum = $idNumberSum % 11;
    if ( $valid_checksum == 10 ) {
        $valid_checksum = 0;
    }
    if ( $checksum == $valid_checksum ) {
        return true;
    }
    return false;
}

The following method will allow you to get the citizen birth date from the ID:

/**
* Get the birthday date from given valid ID of Bulgarian citizen (EGN).
* @author Samui Banti - https://samiwell.eu
* @param string $idNumber - A unique ID of Bulgarian citizen.
* @return string $birthDay - The birth date in format YYYY-MM-DD
*/
function getBirthDayFromBulgarianID($idNumber) 
{
    $year   = substr($idNumber, 0, 2);
    $month  = substr($idNumber, 2, 2);
    $day    = substr($idNumber, 4, 2);
    
    if ($month > 40) {
        $month -= 40;
        $year += 2000;
    } else if ($month > 20) {
        $month -= 20;
        $year += 1800;
    } else {
        $year += 1900;
    }
    $month = str_pad($month, 2, '0', STR_PAD_LEFT);
    $day = str_pad($day, 2, '0', STR_PAD_LEFT);
    $birthDay = "{$year}-{$month}-{$day}";
    return $birthDay;
}