The horizontal equivalent to this function is called transverseImage()
    (PECL imagick 2, PECL imagick 3)
Imagick::flipImage — Creates a vertical mirror image
Creates a vertical mirror image by reflecting the pixels around the central x-axis.
此函数没有参数。
   成功时返回 true。
  
错误时抛出 ImagickException。
Example #1 Imagick::flipImage()
<?php
function flipImage($imagePath) {
    $imagick = new \Imagick(realpath($imagePath));
    $imagick->flipImage();
    header("Content-Type: image/jpg");
    echo $imagick->getImageBlob();
}
?>
The horizontal equivalent to this function is called transverseImage()
    
To avoid the x-axis and y-axis discussion, flipping emulates a mirror at the bottom (or top) of the image. 
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They mean, "Creates a horizontal mirror image by reflecting the pixels around the central y-axis."