<?php

function getUTF16CodeUnits($string) {
    $string = substr(json_encode($string), 1, -1);
    preg_match_all("/\\\\u[0-9a-fA-F]{4}|./mi", $string, $matches);
    return $matches[0];
}

function JS_charCodeAt($utf16CodeUnitsArray, $index) {
    $unit = $utf16CodeUnitsArray[$index];

    if(strlen($unit) > 1) {
        $hex = substr($unit, 2);
        return hexdec($hex);
    }
    else {
        return ord($unit);
    }
}

$str = "t🙏🏿😘🎚↙️🕗🇨🇬芳";
$str = str_repeat($str, 250); //repeat $str 250 times

$utf16CodeUnits = getUTF16CodeUnits($str);

echo "Length is: ".count($utf16CodeUnits)."\n";
for($i=0; $i<count($utf16CodeUnits); $i++) {
    echo "charCodeAt(".$i."): ".JS_charCodeAt($utf16CodeUnits, $i)."\n";
}