PHP substr波斯语字符串以问号结尾

Question

In PHP when I want to substr the Persian string 'حروف فارسی برای امتحان', and it appends the question mark at the end of string.

This is my code:

$a = 'حروف فارسی برای امتحان'; echo substr($a,0,10);

And the problem is حروف �

Anyone has any suggestion or solution for this regards? much appreciated.