Your NJ credit for the Philadelphia Wage Tax cannot be more than the amount you would have paid if you earned the income in New Jersey.
http://www.state.nj.us/treasury/taxation/njit14.shtml
Because your NJ gross income is less than $10,000, you do not need to file a NJ return. Since your NJ taxes are zero, your credit for Philadelphia taxes paid is also zero.
http://www.state.nj.us/treasury/taxation/njit23.shtml
You cannot get a refund from the City of Philadelphia, since you did indeed work there.
