php imagettftext行高

I have successfully written text to my img and positioned it correctly, however the problem I have ran into is that increasing the font size makes the text go off the image.

So I know I can achieve the look I want by just simply increasing the line height of the text writing to the screen. I can't seem to find any documentation on if/how this can be done.

My php code

<?php  

  $font = 'font-type.ttf';

  $rImg = ImageCreateFromJPEG( "test.jpg" );

  $color = imagecolorallocate($rImg, 255, 255, 255);

  //create bounding box
  $bbox = imagettfbbox ( 38 , 0 , $font , urldecode($_GET['promo']) );

  //setting the cordinates
  $x = $bbox[0] + (imagesx($rImg) / 2) - ($bbox[4] / 2) + 200;
  $y = $bbox[1] + (imagesy($rImg) / 2) - ($bbox[5] / 2) + 25;

  //write it
  imagettftext($rImg, 38, 0, $x, $y, $color, $font, urldecode($_GET['promo']));

  header('Content-type: image/jpeg');
  imagejpeg($rImg, NULL,100);

  imagedestroy($rImg);

?>

I have achieved my desired look using this function

function ImageTTFTextWithTracking($im, $size, $angle, $t, $x, $y, $color, $font, $text) {
$numchar = strlen($text);
for($i = 0; $i < $numchar; $i++) {
    # Assign character
    $char[$i] = substr($text, $i, 1);

    # Write character
    imagettftext($im, $size, $angle, ($x + $w + ($i * $t)), $y, $color, $font, $char[$i]);

    # Get width of character
    $width = imagettfbbox($size, $angle, $font, $char[$i]);
    $w = $w + $width[2];
}
}
ImageTTFTextWithTracking($rImg, 48, 0, 10, $x, $y, $colorWhite, $font, urldecode($_GET['code']));