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.
Applys an arithmetic, relational, or logical expression to an image. Use these operators to lighten or darken an image, to increase or decrease contrast in an image, or to produce the "negative" of an image.