How to annotate with largest font size possible when given a height of a (blank) image?

I'm using PHP/imagemagick to annotate an image. But I have a constraint of how big the font can be.

for example... given the text string "hello world", font "courier", and a height of 150px, how do I create a transparent background PNG with height 150x that has the largest font size开发者_JAVA百科 possible with text "hello world" in courier?

Is there a way to do this w/o incrementally "testing" to see if the font is too big?

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Not really since the letter kerning of every font is unique and can even vary on different operating systems.

Using a monospaced font might make it predictable enough to guess the right width but thats probably not an option...

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You need to get the width of the font. You can use imagefontwidth($font) to do this. If you look through the comments you will an example of someone trying to get a width of text.