Convert 10 OR 9 Digit ISBN Number to 13 Digit ISBN Number in PHP

$isbn = '178629298X';

/**
* Function accepts either 12 or 13 digit number, and either provides or checks the validity of the 13th checksum digit
*    Optionally converts to ISBN 10 as well.
*/
function isbn13checker($input, $convert = FALSE){
$output = FALSE;
if (strlen($input) < 12){
$output = array('error'=>'ISBN too short.');
}
if (strlen($input) > 13){
$output = array('error'=>'ISBN too long.');
}
if (!$output){
$runningTotal = 0;
$r = 1;
$multiplier = 1;
for ($i = 0; $i < 13 ; $i++){
$nums[$r] = substr($input, $i, 1);
$r++;
}
$inputChecksum = array_pop($nums);
foreach($nums as $key => $value){
$runningTotal += $value * $multiplier;
$multiplier = $multiplier == 3 ? 1 : 3;
}
$div = $runningTotal / 10;
$remainder = $runningTotal % 10;

$checksum = $remainder == 0 ? 0 : 10 - substr($div, -1);

$output = array('checksum'=>$checksum);
$output['isbn13'] = substr($input, 0, 12) . $checksum;
if ($convert){
$output['isbn10'] = isbn13to10($output['isbn13']);
}
if (is_numeric($inputChecksum) && $inputChecksum != $checksum){
$output['error'] = 'Input checksum digit incorrect: ISBN not valid';
$output['input_checksum'] = $inputChecksum;
}
}
return $output;
}

/**
* Function accepts either 10 or 9 digit number, and either provides or checks the validity of the 10th checksum digit
*    Optionally converts to ISBN 13 as well.
*/
function isbn10checker($input, $convert = FALSE){
$output = FALSE;
if (strlen($input) < 9){
$output = array('error'=>'ISBN too short.');
}
if (strlen($input) > 10){
$output = array('error'=>'ISBN too long.');
}
if (!$output){
$runningTotal = 0;
$r = 1;
$multiplier = 10;
for ($i = 0; $i < 10 ; $i++){
$nums[$r] = substr($input, $i, 1);
$r++;
}
$inputChecksum = array_pop($nums);
foreach($nums as $key => $value){
$runningTotal += $value * $multiplier;
//echo $value . 'x' . $multiplier . ' + ';
$multiplier --;
if ($multiplier === 1){
break;
}
}
//echo ' = ' . $runningTotal;
$remainder = $runningTotal % 11;
$checksum = $remainder == 1 ? 'X' : 11 - $remainder;
$checksum = $checksum == 11 ? 0 : $checksum;
$output = array('checksum'=>$checksum);
$output['isbn10'] = substr($input, 0, 9) . $checksum;
if ($convert){
$output['isbn13'] = isbn10to13($output['isbn10']);
}
if ((is_numeric($inputChecksum) || $inputChecksum == 'X') && $inputChecksum != $checksum){
$output['error'] = 'Input checksum digit incorrect: ISBN not valid';
$output['input_checksum'] = $inputChecksum;
}
}
return $output;
}

function isbn10to13($isbn10){

$isbnStem = strlen($isbn10) == 10 ? substr($isbn10, 0,9) : $isbn10;
$isbn13data = isbn13checker('978' . $isbnStem);
return $isbn13data['isbn13'];

}

function isbn13to10($isbn13){

$isbnStem = strlen($isbn13) == 13 ? substr($isbn13, 12) : $isbn13;
$isbnStem = substr($isbn13, -10);
$isbn10data = isbn10checker($isbnStem);
return $isbn10data['isbn10'];
}
echo isbn10to13($isbn);