good
Array ( [0] => 1234567891 [1] => 123 [2] => 456 [3] => 7891 ) Valid number

show code

<?php
if ($_GET['phone'])
{
    $match = preg_match("/(\d{3})(\d{3})(\d{4})/", $_GET['phone'], $matches);
    echo ($match) ? "good<br>" : "bad<br>";
    print_r($matches);
    echo ($match) ? "Valid number" : 'Invalid number';
}
?>