test.php

<?php

/**
 * Mimics the behaviour of javascript's charCodeAt.
 *
 * @param string $x
 * @param int $index
 *
 * @return int
 */
function charCodeAt(string $x, int $index): int {
	$x = mb_convert_encoding($x, "UTF-16", "UTF-8");

	if((($index * 2) + 1) > strlen($x)) {
		throw new \InvalidArgumentException("Index of short ({$index}) exceeds the size of the converted string (".strlen($x)." bytes).");
	}

	// Unpack unsigned short (2 bytes / 16 bits)
	$a = ord($x{$index * 2});
	$b = ord($x{($index * 2) + 1});

	return ($a * (2 ** 8)) + $b;
}