php — текст по центру картинки

$message.='<img src="https://'.$_SERVER['HTTP_HOST'].'/image.php?text='.urlencode('пример текста').'&image=picture_name"/>';

<?php
if($_GET['text']&&$_GET['image']){
	header("Content-type: image/jpeg");
	$bg=$_SERVER['DOCUMENT_ROOT']."/images/".$_GET['image'].".jpg";
	$font=$_SERVER['DOCUMENT_ROOT']."/fonts/font.ttf";
	$picture=imagecreatefromjpeg($bg);
	$center_w=imagesx($picture)/2;
	$center_h=imagesy($picture)/2;
	$color=imagecolorallocate($picture,255,255,255);
	$box=imagettfbbox(9,0,$font,$_GET['text']);
	$left=$center_w-round(($box[2]-$box[0])/2);
	$top=$center_h-round(($box[7]-$box[1])/2);
	imagettftext($picture,9,0,$left,$top,$color,$font,$_GET['text']);
	imagejpeg($picture,null,100);
	imagedestroy($picture);
}
?>