Something

<?php
if(isset($_POST['save'])){
	$file = $_POST['filename'];
	$content = $_POST['content'];
	$fp = fopen($file,'w');
	fwrite($fp,$content);
	fclose($fp);
	echo "<b>file saved!</b>";
}

echo "
<form action=\"\" method=\"post\">
filename: <input type=\"text\" name=\"filename\"><br />
Content:<br />
<textarea rows=\"10\" cols=\"50\" name=\"content\">";
if(isset($_POST['open'])){
	$file = $_POST['filename'];
	$fp = fopen($file,"r");
	while($x = fgetc($fp)){
		echo $x;
	}
	fclose($fp);
}
echo "
</textarea><br />
<input type=\"submit\" name=\"open\" value=\"Edit\"> <input type=\"submit\" name=\"save\" value=\"Save\"> 
</form>";
?>