php-questions about checking file type functions

Php

 /** * Check File type * * @access public * @param string filename * @param string Realna Me real file name * @param string limit_ext_types allowed file types * @return string */function Check_file_typ  E ($filename, $realname = ', $limit _ext_types = ') {if ($realname) {$extname = Strtolower (substr ($realname, Strrpos ($realname, '. ') + 1);//Gets the file suffix name} else {$extname = Strtolower (substr ($filename, Strrpos ($filenam E, '. ')    + 1)); if ($limit _ext_types && stristr ($limit _ext_types, ' | ' $extname. '|')    = = = False)//??    {return ';    } $str = $format = ";    $file = @fopen ($filename, ' RB ');    if ($file) {$str = @fread ($file, 0x400);//Read the first 1024 bytes @fclose ($file); } else {if (Stristr ($filename, root_path) = = = False) {if ($extname = = ' jpg ' | | $extname = = ' JPEG ' | | $extname = = ' gif ' | | $extname = = ' png ' | |             $extname = = ' Doc ' | |   $extname = = ' xls ' | | $extname = = ' txt ' | | $extname = = ' Zip ' | | $extname = = ' rar ' | |                $extname = = ' ppt ' | | $extname = = ' pdf ' | | $extname = = ' RM ' | | $extname = = ' Mid ' | | $extname = = ' wav ' | |                $extname = = ' bmp ' | | $extname = = ' swf ' | | $extname = = ' CHM ' | | $extname = = ' SQL ' | | $extname = = ' Cert ' | |                 $extname = = ' Pptx ' | | $extname = = ' xlsx ' | |            $extname = = ' docx ') {$format = $extname;        }} else {return '; }} if ($format = = ' && strlen ($STR) >= 2) {if (substr ($str, 0, 4) = = ' MThd ' && $ext        Name! = ' txt ') {$format = ' mid ';        } elseif (substr ($str, 0, 4) = = ' RIFF ' && $extname = = ' wav ') {$format = ' wav ';        } elseif (substr ($str, 0, 3) = = "\xff\xd8\xff") {$format = ' jpg '; } elseif (substr ($str, 0, 4) = = ' GIF8 ' && $extname! = 'TXT ') {$format = ' gif ';        } elseif (substr ($str, 0, 8) = = "\x89\x50\x4e\x47\x0d\x0a\x1a\x0a") {$format = ' png ';        } elseif (substr ($str, 0, 2) = = ' BM ' && $extname! = ' txt ') {$format = ' bmp ';            } ElseIf ((substr ($str, 0, 3) = = ' CWS ' | | substr ($STR, 0, 3) = = ' FWS ') && $extname! = ' txt ') {        $format = ' swf ';            } elseif (substr ($str, 0, 4) = = "\xd0\xcf\x11\xe0") {//d0cf11e = = Docfile = Microsoft Office Document if (substr ($str, 0x200,4) = = "\xec\xa5\xc1\x00" | | $extname = = ' Doc ') {$format = ' doc '            ;             } elseif (substr ($str, 0x200,2) = = "\x09\x08" | | $extname = = ' xls ') {$format = ' xls ';  } elseif (substr ($str, 0x200,4) = = "\xfd\xff\xff\xff" | | $extname = = ' ppt ') {$format            = ' ppt '; }} elseif (substr ($str, 0, 4) == "pk\x03\x04") {if (substr ($str, 0x200,4) = = "\xec\xa5\xc1\x00" | | $extname = = ' docx ') {            $format = ' docx '; } elseif (substr ($str, 0x200,2) = = "\x09\x08" | | $extname = = ' xlsx ') {$format = ' xlsx            ';            } elseif (substr ($str, 0x200,4) = = "\xfd\xff\xff\xff" | | $extname = = ' pptx ') {$format = ' pptx ';            }else {$format = ' zip ';        }} elseif (substr ($str, 0, 4) = = ' rar! ' && $extname! = ' txt ') {$format = ' Rar ';        } elseif (substr ($str, 0, 4) = = "\x25pdf") {$format = ' pdf ';        } elseif (substr ($str, 0, 3) = = "\x30\x82\x0a") {$format = ' cert ';        } elseif (substr ($str, 0, 4) = = ' ITSF ' && $extname! = ' txt ') {$format = ' chm ';        } elseif (substr ($str, 0, 4) = = "\x2ermf") {$format = ' rm '; } elsEIF ($extname = = ' sql ') {$format = ' sql ';        } elseif ($extname = = ' txt ') {$format = ' txt '; }} if ($limit _ext_types && stristr ($limit _ext_types, ' | ' $format. '|')    = = = False) {$format = '; } return $format;}

The entire function is used to detect the file type, where

 elseif (substr($str ,0, 3) == "\xFF\xD8\xFF")        {            $format = 'jpg';        }                 elseif (substr($str ,0, 8) == "\x89\x50\x4E\x47\x0D\x0A\x1A\x0A")        {            $format = 'png';        }                 elseif (substr($str ,0, 4) == "\xD0\xCF\x11\xE0")                   if (substr($str,0x200,4) == "\xEC\xA5\xC1\x00" || $extname == 'doc')            {                $format = 'doc';            }            elseif (substr($str,0x200,2) == "\x09\x08" || $extname == 'xls')            {                $format = 'xls';            } elseif (substr($str,0x200,4) == "\xFD\xFF\xFF\xFF" || $extname == 'ppt')            {                $format = 'ppt';            }

What are the principles of these statements? And in hexadecimal for everything?

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