GD库，php，如何使用文本自动设置大小

Question

I have a proplem.

How can I set the size automatically with text When I change the font size and text space

$font = 'Helvetica.ttf';
$text ='Test';
$size = '12';
//-------------------------++
$im = imagecreatetruecolor(150, 25);
$color = imagecolorallocate($im, 119,119,119); //http://www.color-hex.com/color/777777
$transparent = imageColorAllocateAlpha($im, 0, 0, 0, 127);
//-------------------------++
imageSaveAlpha($im, true);
imageAlphaBlending($im, false);
imagefilledrectangle($im, 0, 0, 399, 29, $transparent);
imageAlphaBlending($im, true);
//-------------------------++
$bbox = imagettfbbox($size, 0, $font, $text);
$dx = ($bbox[2]-$bbox[0])/2.0 - ($bbox[2]-$bbox[4])/2.0;
$dy = ($bbox[3]-$bbox[1])/2.0 + ($bbox[7]-$bbox[1])/2.0;
$x = 0;
$y = 0;
$px = $x-$dx;
$py = $y-$dy;
//-------------------------++
imagettftext($im, $size, 0, $px, $py, $color, $font, $text);
//-------------------------++
header('Content-Type: image/png');
imagePng($im);
imageDestroy($im);

Problems at the location of the text!