This property specifies the color to use as the transparent color when rendering the image. It replaces the current IMAGETRANSPARENT and TRANSPARENTCOLOR properties, both of which have been deprecated.
This property should be a valid RGB color value or one of the following special values:
| Value | Description |
|---|---|
| "-1" | Use the color of the top left pixel as the color-key. |
| "-2" | Use the color of the top-right pixel as the color-key. |
| "-3" | Use the color of the bottom-left pixel as the color-key. |
| "-4" | Use the color of the bottom-right pixel as the color-key. |
| "-5" | Do not use any color-keying. |