Ajax :: Cannot Pass String To Function

I am using ajax to pass php variable to javasccript function as shown below for my particular application.

i am using the foolowing function from ajax.js freely available from net to implement the ajax functionality as

var abc = new sack();
PHP portion which passes the response text:
************************************
<?
$table=$_GET['tablename'];

[Code]....

1] When $table is having a string with only numeric characters for eg: "123" we get the alert

2] When $table is having a string with alphanumeric characters for eg: "123efg" we get the error report as "missing ) after argument list"

3] When $table is having a string with alphanumeric characters for eg: "efg123" or "abcdfr" we get the error report as "efg123 is not defined" or "abcdfr is not defined".