This function is currently not documented; only the argument list is available.
This function is EXPERIMENTAL. The behaviour of this function, the name of this function, and anything else documented about this function may change without notice in a future release of PHP. Use this function at your own risk.
Applies a special effect to the image, similar to the effect achieved in a photo darkroom by selectively exposing areas of photo sensitive paper to light. Threshold ranges from 0 to QuantumRange and is a measure of the extent of the solarization.