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; 
} 
?>