problem with sending AT command in php

[a dehqan <dehqan65@xxxxxxxxx>]

In The Name Of Allah The compassionate merciful

Hello ;
Am trying to send AT commands to my GSMmodem device , in linux with PHP
Am sure it is /dev/ttyusb1
But with [URL="http://pastebin.com/aut4ygXf"]this code[/URL] device respond
the same command and code prints that command
e.g if we send ATD00 it responds ATD00 , or with AT it responds AT instead
of Ok, why ?

Regards dehqan