Contact lookup via Email field

I am using Force.com Toolkit for PHP (Version 20.0) to integrate with Salesforce.

I would like to lookup some Contact via the email field and print in on page if the condition is met. Here is the query I used:

SELECT Name, Email, npe01__HomeEmail__c, npe01__WorkEmail__c, npe01__AlternateEmail__c FROM Contact WHERE Email = "a@a.com"

In Workbench everything works fine, however, when I use the same query in PHP I get the following error:

'MALFORMED_QUERY: npe01__AlternateEmail__c FROM Contact WHERE Email="a@a.com"
ERROR at Row:1:Column:112
Bind variables only allowed in Apex code'

What would be best practice to help me solve this problem?

Thanks!