It allows you to return plain text from your PHP script, and then have a browser JavaScript parse the returned text using the eval() function and turn it directly into a usable javascript object, array or variable. Say your PHP script returns this in the responseText of the AJAX call: {name: 'Barry', age: 31} In your callback javascript function: <i> </i>var respObj = eval('('+responseText+')'); alert('Name: '+respObj.name); alert('Age: '+respObj.age);