Replacing tag in file

I'm stuck. Please help.

I have a .html file that I would like to open, replace certain texts or tags, and write it back to the file.

Just for testing. I have the following information in
test_file.html

test1
test2
test3

I would like to replace "test2" with "PHP". Below is my code, and it doesn't seem to work.

Any help is much appreciated.

<?
$fd = fopen("test_file.html", "r");
while (!feof($fd))
{
$buffer .= fgets($fd, 4096);
echo $buffer;
}
fclose($fd);

str_replace("test2","PHP",$buffer);

$fd = fopen("/web123/demo/test_file.html", "wb");
$done = fputs($fd,$buffer);
fclose($fd);
?>

there is 3 examples for u to check your coding:
i think u may have wrong in 1 .

1.
<?php
$fd = fopen("/tmp/myfile.txt", "r");
while ($buffer = fgets($fd, 4096)) {
echo $buffer;
}
fclose($fd);
?>

2.
<?php
$bodytag = str_replace("%body%", "black", "<body text=%body%>");
echo $bodytag;
?>

$mytempfile=fopen("magzinecontentfile.html","w");
fputs($mytempfile,$one["magzine_content"]);
fclose($mytempfile);

icq: 21083512