This function is deprecated from last stable version (3.4.4 - 2019-05-02) of PECL Imagick extension.
More info: http://pecl.php.net/package-changelog.php?package=imagick&release=3.4.4
"setImageAlpha" is the new alternative function, but doesn't work with ImageMagick < 7.
More info: https://github.com/avalanche123/Imagine/issues/715