function file_size($file){  
$size=filesize($file);  
if($size >= 1073741824){  
$size=round($size/1073741824)."Gb";  
}  
elseif($size >= 1048576){  
$size=round($size/1048576)."Mb";  
}  
elseif($size >= 1024){  
$size=round($size/1024)."Kb";  
}else{  
$size=$size. "b";  
}  
echo "File size: <b>$size</b>" ;  
}  

file_size($file="index.php");