php怎么判断图片类型

$filename = '617.gif' ; 
function pictype ( $file ) 
{ 
   /*$png_header = "/x89/x50/x4e/x47/x0d/x0a/x1a/x0a"; 
   $jpg_header = "/xff/xd8";*/
   $header = file_get_contents ( $file , 0 , NULL , 0 , 5 ); 
   //echo bin2hex($header); 
   if ( $header { 0 }. $header { 1 }== "/x89/x50" ) 
   { 
     return 'png' ; 
   } 
   else if( $header { 0 }. $header { 1 } == "/xff/xd8" ) 
   { 
     return 'jpeg' ; 
   } 
   else if( $header { 0 }. $header { 1 }. $header { 2 } == "/x47/x49/x46" ) 
   { 
     if( $header { 4 } == "/x37" ) 
       return 'gif87' ; 
     else if( $header { 4 } == "/x39" ) 
       return 'gif89' ; 
   } 
} 
echo pictype ( $filename );