Hello Dolly,
Sorry, I had not looked at that thread so I didn't saw your file.
For me it's working perfectly and characters are printed as expected
See this screenshot in acrobat reader
the blue line is produced by
$textn = 'This is text hardcoded in certificate: çÇğĞüÜöÖıIşŞ';
certificate_print_text($pdf, $x, $y, 'C', 'freeserif','', 30, $textn);
And the line at the bottom is produced by
certificate_print_text($pdf, $custx, $custy, 'C', 'freeserif','' , 25, $certificate->customtext);
Of course I have typed "This is customtext: çÇğĞüÜöÖıIşŞ" when I created the certificate.
So no problem for me. I don't understand why it is not working for you.