asterisk users - Oct 2010 - How to execute Asterisk Functions in PHPAGI

Hi, all
        I can use $agi->exec() to excute applications in Asterisk.
such as $agi->exec("Set",abc=1)

But how could i excute Asterisk Functions use agi functions?  for example:

I want to excute SHARED function in PHPAGI. i use
$agi->exec("SHARED",callernum) to do it.

but failed with info:

<SIP/321-00000747>AGI Rx << EXEC SHARED callernum
    -- AGI Script Executing Application: (SHARED) Options: (callernum)
[Oct 19 00:06:13] WARNING[2638]: res_agi.c:1757 handle_exec: Could not
find application (SHARED)

Could you help me?
-- 
Appreciate your kindly advise and help.
Thanks & Regards
Sucan