function dl_file($file){
     //这是下载文件的函数,$file是文件路径.  //First, see if the file exists  if (!is_file($file)) { die("404 File not found!"); } //Gather relevent info about file $len = filesize($file); //则返回文件大小的字节数。若失败，则返回 false 并生成一条 E_WARNING 级的错误。 $filename = basename($file); //返回路径中的文件名部分。 //图片的格式（JPG）换成小写 strtolower（把所有大写换成小写） $file_extension = strtolower(substr(strrchr($filename,"."),1)); //strrchr() 函数（在php中）查找字符在指定字符串中从左面开始的最后一次出现的位置，如果成功，返回该字符以及其后面的字符，如果失败，则返回 NULL。 //This will set the Content-Type to the appropriate setting for the file switch( $file_extension ) { case "pdf": $ctype="application/pdf"; break; break是结束整个循环体，continue是结束单次循环 case "exe": $ctype="application/octet-stream"; break; case "zip": $ctype="application/zip"; break; case "doc": $ctype="application/msword"; break; case "xls": $ctype="application/vnd.ms-excel"; break; case "ppt": $ctype="application/vnd.ms-powerpoint"; break; case "gif": $ctype="image/gif"; break; case "png": $ctype="image/png"; break; case "jpeg": case "jpg": $ctype="image/jpg"; break; case "mp3": $ctype="audio/mpeg"; break; case "wav": $ctype="audio/x-wav"; break; case "mpeg": case "mpg": case "mpe": $ctype="video/mpeg"; break; case "mov": $ctype="video/quicktime"; break; case "avi": $ctype="video/x-msvideo"; break; case "php": case "htm": case "html": case "txt": die("Cannot be used for ". $file_extension ." files!"); break; default: $ctype="application/force-download"; }