Ben de ajax ile nasıl yapılır onu yazayım. :)

<input type="text" name="ad" required  />
<input type="text" name="soyad" required  />
<input type="text" name="kullaniciadi" required  />

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.6.0/jquery.min.js"></script>
<script>
	$(function() {
		$("input[name=ad],input[name=soyad]").on("keyup change", function(e) {
			var ad = $("input[name=ad]").val();
			var soyad = $("input[name=soyad]").val();
			$.ajax({
				url: "post.php",
				type: "POST",
				data: {ad: ad, soyad:soyad},
				success: function(data) {
					$("input[name=kullaniciadi]").val(data);
				}
			});
		});
	})
</script>

post.php sayfası.

<?php 
	
	$ad = $_POST["ad"];
	$soyad = $_POST["soyad"];

	$username = $ad.$soyad;

	echo sefLink($username);

	function sefLink($param)
	{
		$convert_from 	= array("Ç", "ç", "Ğ", "ğ", "ı", "İ", "Ö", "ö", "Ş", "ş", "Ü", "ü");
		$convert_to 	= array("C", "c", "G", "g", "i", "I", "O", "o", "S", "s", "U", "u");
		$param 			= trim($param);
		$param 			= str_replace($convert_from,$convert_to,$param);
		$param 			= preg_replace('/\r|\n/',' ',$param);
		$param 			= preg_replace('/[^A-Za-z0-9]/',' ',$param);
		$param 			= preg_replace('/\s+/',' ',$param);
		$param 			= str_replace('/\s+/',' ',$param);
		$param 			= str_replace(' ','-',$param);
		$param 			= strtolower($param);
		return $param;
	}

?>