How to make word wrap limited by stringlength in pixels (PHP) - image

Question

Im trying to sign an image with GD.
I have a line of text, which may look like this: "This is a very long line of text"
If I use 60th font, it wont fit in my image, so I need some how do wordwrap
Now I'm using loop that is doing word wrap limited by number of symbols and adding text to image line by line..
$words = explode(" ", $text);
if (strlen($words[0]) > 20)
$output = substr($words[0], 0, 20);
else {
$output = array_shift($words);
while (strlen($output . " " . $words[0]) <= 20) {
$output .= " " . array_shift($words);
}
}
$text = $str2 = substr($text, strlen($output));
list($left,, $right) = imageftbbox($font_sizet, 0, $font_path, $output);
$width = $right - $left;
$leftx = (740 - $width) / 2;
$topy = (560 + $i * 30);
imagettftext($img, $font_sizet, 0, $leftx, $topy, $white, $font_path, $output);
The problem with this script is that if the first letters are going to be "WWWWWWWWWWW", the image will fit only 10 symbols... and if first letters are going to be "llllllllllll" the text will look short.
The width of text I cant determine by using this line:
list($left,, $right) = imageftbbox( $font_sizet, 0, $font_path, $output);
$width = $right - $left;
<- i think that somehow I can use this $width in my
loop to compare with maxlength
How can I change limiting by number of symbols to limiting by number of pixels (lets say, I want the line to be no longer than 300px)?