PHP wamp using relative path failed to write to text document - php

Question

kindly help me.
my env as follow : window 7 x64,wamp x64 v2.5.
apache web root directory : D:/wampProject/.
I want to write to the target text file in D:/wampProject/chapter02/orders.txt
syntaxTest.php in D:/wampProject/,as below.
<?php
header("Content-type: text/html; charset=utf-8");
$document_root= $_SERVER['DOCUMENT_ROOT'];
//#1 $filename = "$document_root"."chapter02/orders.txt"; //works
//#2 $filename="D:/wampProject/"."chapter02/orders.txt"; //works
//#3 $filename="$document_root../orders.txt";
//#3 output: filename D:/wampProject/../orders.txt
//showed contents added in the page after refresh,
//but text file didn't get written refer pics below .
//Got the wrong dir? seems like page result shows 'cache' written content
//added by absolute path in all time by the script.
//#4 $filename="D:/wampProject/"."../orders.txt";
//filenameD:/wampProject/../orders.txt as failed as #3.
if (is_writable($filename)) {
echo 'The file is writable';
} else {
echo 'The file is not writable';
} ;
echo '<br/>';
if (is_readable($filename)) {
echo 'The file is readable';
} else {
echo 'The file is not readable';
};
$fp=fopen($filename,'ab');
$outputstring="address string"."\n";
echo $outputstring.'<br />';
flock($fp,LOCK_EX);
fwrite($fp,$outputstring,strlen($outputstring));
flock($fp,LOCK_UN);
fclose($fp);
echo "fclosed".'<br />';
$filecon=file_get_contents($filename);
echo $filecon."<br />";
?>
page result
orders.txt after using 4 filename paths above