The source contain already <br> tag, or do you apply nl2br() function?
If so you can first do htmlentities() before nl2br():
nl2br(htmlentities($mytextareacontent));
If you are viewing a source of html, i think you have not to convert <br>, but display it: it's source of html.
However if you don't want convert it, just apply htmlentities($mytext) and then str_replace("<br>","<br>",$mytext);
i hope this help you