Why this can NOT merge file more than 4MB? would you fix it?

Why this can NOT merge file more than 4MB?
would you fix it?

The merge method could be rewritten to not read the all parts into memory.

/*------------------------------------------------------------------
- MERGE -
- This function merges splited files that are splited with above -
- split_file function. -
--------------------------------------------------------------------*/
function merge_file($merged_file_name,$parts_num)
{
$content='';
//put splited files content into content
$mhandle=fopen($merged_file_name, 'wb') or die("error creating/opening merged file");
for($i=0;$i<$parts_num;$i++)
{
$file_size = filesize('splited_'.$i);
$handle = fopen('splited_'.$i, 'rb') or die("error opening file");
$content = fread($handle, $file_size) or die("error reading file");
fclose($handle);
//write content to merged file
fwrite($mhandle, $content) or die("error writing to merged file");
}
fclose($mhandle);
return 'OK';
}//end of function merge_file