php function to detect file type

//Detect file type
//by http://bbs.it-home.org
function checkFileType($filename){
 //File header
 $_typecode = array(
 '3780',//PDF
 '8075',//.docx,.xlsx,.pptx,.potx,.vsdx,.odt
 '208207',//.doc,.xls,.ppt,.vsd, .pot,.wps,.dps,.et
 );
 $file = fopen($filename, "rb");
 //contents = stream_get_contents($file);
 //$contents = fread($file, filesize($filename));
 $bin = fread($file, 2); //Read only 2 bytes
 fclose($file);
 $strInfo = @unpack("C2chars", $bin);// C is an unsigned integer. All those found on the Internet are c, which is a signed integer. This will cause abnormal judgment of negative numbers
 $typeCode = intval($strInfo['chars1'].$strInfo['chars2']);
 exec("file $filename",$output,$return_var);//Use the Linux system command file to determine the type of uploaded file, mainly to determine the txt, rtf file type
 $pattern = '/text,/';//rtf and txt documents will be detected as text using file detection
 $_count = preg_match($pattern,strrchr($output[0],":"));
 echo $typeCode;
 if(in_array($typeCode,$_typecode) || $_count == 1) {
 return true;
 }else{
 return false;
 }
 }
 ?>