Use [man]file_get_contents/man instead of [man]file/man and output $prevalue instead of $prevalue[0]. Or, if that's all you need the contents of that file for, just [man]readfile/man it where you need it:
<textarea name="headline" style="width:100%; height:600px"><?php readfile("./headline.txt"); ?></textarea>
If you really want $prevalue to be an array of lines from that file, then implode it for the output:
<textarea name="headline" style="width:100%; height:600px"><?php echo implode('', $prevalue); ?></textarea>
PS: You should get into the habit of using "<?php" instead of "<?", for better script portability and for forward compatibility should the short open tags every be removed from future PHP versions.