Add this code where you want to steal the skin of $sender, change the path at the imagepng method to your target

PM4_SkinStealer.php

<?php

$height = 64;
$width = 64;
switch (strlen($sender->getSkin()->getSkinData())) {
    case 64 * 32 * 4:
        $height = 32;
        break;
    case 64 * 64 * 4:
        break;
    case 128 * 64 * 4:
        $width = 128;
        break;
    case 128 * 128 * 4:
        $height = 128;
        $width = 128;
        break;
}

$img = imagecreatetruecolor($width, $height);
imagealphablending($img, false);
imagesavealpha($img, true);

$index = 0;
for ($y = 0; $y < $height; ++$y) {
    for ($x = 0; $x < $width; ++$x) {
        $list = substr($sender->getSkin()->getSkinData(), $index, 4);
        $r = ord($list[0]);
        $g = ord($list[1]);
        $b = ord($list[2]);
        $a = 127 - (ord($list[3]) >> 1);
        $index += 4;
        $color = imagecolorallocatealpha($img, $r, $g, $b, $a);
        imagesetpixel($img, $x, $y, $color);
    }
}

imagepng($img, "/root/custom.png");
imagedestroy($img);