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An IBAN is validated by converting it into an integer and performing a basic mod-97 operation (as described in ISO 7064) on it. If the IBAN is valid, the remainder equals 1.

The flowing piece of PHP code will preform a equivalent of this check for you and return true for a IBAN with a valid check-sum number.

#!/usr/bin/php
<?php
 
$account = "NL20INGB0001234567";
 
function checkIBAN($iban) {
 
  // Normalize input (remove spaces and make upcase)
  $iban = strtoupper(str_replace(' ', '', $iban));
 
  if (preg_match('/^[A-Z]{2}[0-9]{2}[A-Z0-9]{1,30}$/', $iban)) {
    $country = substr($iban, 0, 2);
    $check = intval(substr($iban, 2, 2));
    $account = substr($iban, 4);
 
    // To numeric representation
    $search = range('A','Z');
    foreach (range(10,35) as $tmp)
      $replace[]=strval($tmp);
    $numstr=str_replace($search, $replace, $account.$country.'00');
 
    // Calculate checksum
    $checksum = intval(substr($numstr, 0, 1));
    for ($pos = 1; $pos < strlen($numstr); $pos++) {
      $checksum *= 10;
      $checksum += intval(substr($numstr, $pos,1));
      $checksum %= 97;
    }
 
    return ((98-$checksum) == $check);
  } else
    return false;
}
 
if (checkIBAN($account))
  echo 'IBAN: '.wordwrap($account, 4, ' ', true)."\n";
else
  echo "No valid IBAN account number\n";
?>

This check does only perform a checksum check. It does not validate country specific characteristics (length and format)