<form name="form" action="save.php" method="post">
<textarea name="mytext" col="10" rows="10"></textarea><br>
<input type="submit" value="Send">
</form>

<?php

if (isset($mytext)) {
$fp = fopen("text.txt","a+");
fwrite($fp,$mytext);
fclose($fp);
unset($mytext);
print "Сохранено.";
}

?>