检测png图片是否完整的php代码

<?php
$filename = ./D243375_0.png;
$filename = realpath($filename);
if (!file_exists($filename)) {
die("图片不存在'!");
}
$size = getimagesize ($filename);
$file_extension = strtolower(substr(strrchr($filename,"."),1));
if("image/png" != $size[mime] || $file_extension != "png"){
die("这不是一张完整的png图片");
}
$img = @imagecreatefrompng ($filename);
if($img){
ob_start("output_handler");
imagepng($img);
ob_end_flush();
}else{
die("不能正确的创建png图形,请检查png图形是否完好'");
}
function output_handler($img) {
header(Content-type: image/png);
header(Content-Length:.strlen($img));
return $img;
}
?>