gamertag_exists.php

<?php
function gamertag_exists($gamertag)
{
    $first_step  = explode("<div id=\"Gamerscore\">", file_get_contents("https://gamercard.xbox.com/en-US/$gamertag.card"));
    $second_step = explode("</div>", $first_step[1]);
    $gamerscore  = $second_step[0];
    if ($gamerscore != "--") {
        return true;
    } else {
        return false;
    }
}
if (gamertag_exists("DaltontasticMC")) {
    echo "This gamertag exists.";
} else {
    echo "This gamertag doesn't exist.";
}
?>