//Either var_export or set print_r to return the output instead of printing it.
$b = array (
'm' => 'monkey',
'foo' => 'bar',
'x' => array ('x', 'y', 'z'));
$results = print_r($b, true); // $results now contains output from print_r
//You can then save $results with file_put_contents. Or return it directly when writing to file:
file_put_contents('filename.txt', print_r($b, true));