"writing" a backspace or del char in a file

Hi i want to erease 'one' or more caracters in a file, so, if this file is too big ( >10MB ) i don't want to read entirely it to substr(); the last line and entirely re-write it... i like to write -at the end by exemple- the char backspace ( 0x08 ) ex : <?php $fp = fopen('some_file','a'); // or 'ab' fputs($fp, sprintf("%c",8)); fclose($fp); ?> but this simply add the char "backspace" at the end and doesn't delete it ! the same problem appears with the character 127 ( delete ). i don't really know if that i want is possible ( perhaps with direct i/o methods ? ) so, please show me the way :)