分割png图片

/***
 * 分割图片
 */
public function cut_img(){
    $filename = 'site_upload/form_file_forinput/20180313/20180313191121824939294.png';
    $p = 3;
    // Get new sizes
    list($width, $height) = getimagesize($filename);
    $newwidth = $width;
    $newheight = floor($height / $p);
    $last = $height % $p;
    // Load
    $source = imagecreatefrompng($filename);
    for( $i=0 ; $i< $p; $i++ ){

        $_p = $newheight*$i;
        if(( $i + 1 ) == $p)
        $newheight += $last;
        $thumb = ImageCreateTrueColor($newwidth, $newheight);
        imagecopyresized( $thumb, $source, 0, 0, 0, $_p, $newwidth,  $height, $width, $height);
        header('Content-type: image/png');
        imagepng( $thumb , "site_upload/form_file_forinput/20180313/t{$i}.png" );
        imagedestroy($thumb);
    }

    echo 'end';exit;
}