Center watermark text on image when unknown size of text - php

Question

I'm trying to create an image with a watermark text in it and I want it to be central on the image. The text I want to watermark can be anywhere from 5 characters to 15 characters, therefore I cannot put a final size for the text to fit every image.
This is the code I use to create the watermarked image
function watermarkImage ($SourceFile, $WaterMarkText, $DestinationFile) {
list($width, $height) = getimagesize($SourceFile);
$image_p = imagecreatetruecolor($width, $height);
$image = imagecreatefromjpeg($SourceFile);
imagecopyresampled($image_p, $image, 0, 0, 0, 0, $width, $height, $width, $height);
$black = imagecolorallocate($image_p, 0, 0, 0);
$font = '../fonts/proxima-nova-light.otf';
$font_size = 100;
imagettftext($image_p, $font_size, 0, 303, 190, $black, $font, $WaterMarkText);
if ($DestinationFile<>'') {
imagejpeg ($image_p, $DestinationFile, 100);
} else {
header('Content-Type: image/jpeg');
imagejpeg($image_p, null, 100);
};
imagedestroy($image);
imagedestroy($image_p);
};
Which does an excellent job on some texts but when I try it on longer texts it looks bad.
I want to - somehow - calculate the optimal size of text and choose size, x and y from there.
Any ideas?

Answer 1

After doing some research with help from #Sammitch I was able to figure it out. Here is the working code:
<?php
function watermarkImage ($SourceFile, $WaterMarkText, $DestinationFile) {
$font = 'fonts/your-font-here.ttf';
$font_size = 40;
list($width, $height) = getimagesize($SourceFile);
$image_p = imagecreatetruecolor($width, $height);
$image = imagecreatefromjpeg($SourceFile);
imagecopyresampled($image_p, $image, 0, 0, 0, 0, $width, $height, $width, $height);
$black = imagecolorallocate($image_p, 0, 0, 0);
$bbox = imagettfbbox($font_size, 0, $font, $WaterMarkText);
$x = $bbox[0] + (imagesx($image) / 2) - ($bbox[4] / 2) + 10;
$y = $bbox[1] + (imagesy($image) / 2) - ($bbox[5] / 2) - 5;
imagettftext($image_p, $font_size, 0, $x, $y, $black, $font, $WaterMarkText);
if ($DestinationFile<>'') {
imagejpeg ($image_p, $DestinationFile, 100);
} else {
header('Content-Type: image/jpeg');
imagejpeg($image_p, null, 100);
};
imagedestroy($image);
imagedestroy($image_p);
};
?>

Answer 2

Calculate the position of the watermark in the output image (the
watermark shall be placed in the Center of the Image)
$watermark_pos_x = (imagesx($image)/2) - (imagesx($watermark)/2) - 15;
$watermark_pos_y = (imagesy($image)/2) - (imagesy($watermark)/2) - 10;
Use this value on function.