Execute text2wav file options from php file

I have tired the below line for generating text2wav file from php file to increase the volume its not executing but its working in dialplan(extensions.conf) successfully. pls correct the below syntax and also provide code for changing the default voice options.

$agi=new AGI();

$agi->System(/bin/echo 'Hello,Welcome to all' | /usr/bin/text2wave -scale 1.5 -F 8000 -o /MR/tester.wav)

Thanking you