Description:
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Assuming a subdirectory named "fonts" contains 30 fonts named font_1.ttf - font_30.ttf, calling imagettfbox with a fontname such as "../fonts/font_10.ttf" loads incorrect font, the number part is somehow being altered and the wrong font is loading.

Somehow the combination of ../ and _ in the finename param are screwing things up.  This is not a file/font not found error.  This is the font that loads font_13.ttf is actually loading font_14.ttf, or font_28.ttf is loading font_10.ttf.  I can only guess the underscore is changing the meaning of the number or the number is being converted.

Test script:
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Works:

$box=imagettfbbox(100,0,"fonts/font_10.ttf","some text");

Doesn't work (wrong font loads):

$box=imagettfbbox(100,0,"../fonts/font_10.ttf","some text");

Workaround 1: remove underscore from font names

$box=imagettfbbox(100,0,"../fonts/font10.ttf","some text");

Workaround 2: move app that loads font to higher directory level (no ../ in path)

$box=imagettfbbox(100,0,"fonts/font_10.ttf","some text");