Description:
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The $font parameter in the GD extension function imagettftext() should accept a filename without path, and per the PHP manual GD should search for that font file using a library-defined font path or GDFONTPATH environment variable.

This works, but only if PHP was built without thread-safety. If PHP is built with thread safety, it is unable to find TrueType font files using the search path (GDFONTPATH or the built-in default).

(I think this is happening because when ZTS is defined, VIRTUAL_DIR is defined, and that causes the GD function php_imagettftext_common() to run the font argument through VCWD_REALPATH(). That fails when the font argument is not a complete file path.)

Also, if you build PHP with Apache2 (--with-apx2), you will get a thread-safe PHP if your Apache installation is configured to use the Event MPM or Worker MPM, and a non-thread-safe PHP if your Apache installation is configured to use the Pre-fork MPM. This results in the really strange situation where whether GD can find a TTF font file on the search path depends on which Apache MPM was configured when you built PHP.

Test script:
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<?php
$font = 'Arial.ttf';  // This is for Windows. Change the name for others.
$im = imagecreate(600, 400);
$background_color = imagecolorallocate($im, 255, 255, 255);
$text_color = imagecolorallocate($im, 0, 0, 255);
imagettftext($im, 14, 0, 20, 200, $text_color, $font, 'Font Test');
imagepng ($im);

Expected result:
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Creates a PNG image file on standard output, with the text 'Font Test'.

Actual result:
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When using non-thread-safe PHP, the expected result.

When using thread-safe PHP, a warning message, and no text is drawn: "PHP Warning:  imagettftext(): Invalid font filename in ..."