They are not the same thing.
Diethyl ether: CH3CH2-O-CH2CH3
Petroleum ether: CH3CH2CH2CH2CH2CH3 (and isomers/analogs)
They are both non-polar solvents, so they are both immiscible with water, but diethyl ether is somewhat more polar than pet ether, so most compounds usually has a better solubility in the former.