php 判断是不是为PNG格式的图片

function isPng($pngPath)
{
	$size = getimagesize ($pngPath); 

	$file_extension = strtolower(substr(strrchr($pngPath,'.'),1));

	if('image/png' != $size['mime'] || $file_extension != 'png')
	{ 
		return FALSE; 
	} 
	else {
		return TRUE;
		
	}
	
}