You also may want to consider updating your software. The warnings you are receiving are related to what the PHP authors recommend as best-practice. Downgrading PHP is a band-aid fix as the code is still not best-practice, but the old version of PHP does not warn about this.
The warning message is extremely low priority, meaning it is unlikely to cause a problem, but in future versions of PHP it can theoretically become a higher level error if the function is deprecated, in example.