Fighting with formfeed

Hey, I need your help :))

I currently am trying to get some latex code from a json file on a website (I use MathJax to to display the LaTex there) I verified that the LaTex code read correctly out of the json file and that it works if I hardcode it in the HTML file. My problem (at least I think that is it) is:

To display a fraction the LaTex code is

frac{a}{b} 

when php gets its hands on this, its sees

f rac{a}{b} 

and turns my f into a control char for a formfeed (?!)

is there a way to bypass this?

cheers