Converts each character of a text string to its hexadecimal representation from the Unicode code table. Forms a sequence delimited by space. Speed and efficiency were not tested.

UStringToHex.php

<?php
$string = 'è Ё𐐷-Xπ 🤔';

foreach (preg_split('//u', $string) as $char) {
	if (!$char) continue;

	$hex = base_convert(mb_ord($char, 'UTF-8'), 10, 16);
	$pad = strlen($hex) > 4 ? 8 : 4;
	$hex = str_pad($hex, $pad, '0', STR_PAD_LEFT);

	echo 'U+' . $hex . ' ';
}