Credit Card Validation using the mod10 algorithm in PHP

I’m working on a site that will use the Paypal API for submitting merchant account transactions to them. I’d like to validate as much credit card information as possible before passing any information to a 3rd party, since are different kind of credit cards companies and options, so I’ve been reading to find out more about it. I came across the mod10 check that credit cards use and wrote a little PHP function to validate a card number

function sumdigits($number)
{
  $sum = 0;
  for($i = 0; $i <= strlen($number) - 1; $i++) {
    $sum += substr($number, $i, 1);
  }
  return $sum;
}

function mod10check($number)
{
  $sum_number = '';
  for($i = strlen($number) - 1; $i >= 0; $i--) {
    $thisdigit = substr($number, $i, 1);
    $sum_number .= ( $loop %2 == 0) ? $thisdigit : sumdigits($thisdigit * 2);
  }
  return sumdigits($sum_number) % 10 == 0 ? true : false;
}

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