SetTTFPath — Set the name of the default TrueType font directory
$plot->SetTTFPath($path)
The default TrueType font directory is '.', meaning fonts will only be searched for in the directory containing your main PHP script. This default can be changed at installation time as described in Section 1.2, “Installing”.
Starting with PHPlot-5.0rc3, the default TrueType font directory is used both for the default font set with SetDefaultTTFont and for fonts set with SetFont. Refer to those two functions for details.
See the note at the end of the reference for SetUseTTF on how to set up and use TrueType fonts.