Calling i Function from PHP

I thought I could do this in PHP:

$Wk_Error = MSP_ValidateStoreFN($Wk_Store);

Where MSP_ValidateStoreFN is a function on the i. I'm getting this message
in php.log:

"PHP Fatal error: Call to undefined function MSP_ValidateStoreFN() in ..."

I created the function with the Create Function SQL statement on the i.

Is this syntax not available, and I need to use db2_exec or db2_execute?

I believe I could use this function like this:
"Select * from StrMaster where MSP_ValidateStoreFN($Wk_Store) = 'Y';"
Is that correct?

Thanks!