CodeIgniter: A non well formed numeric value encountered
All you coding is correct. problem is with +2weeks. When you use strtotime() function you need to keep <space> on between text(+2<SPACE>weeks).
Examples(from W3Schools)
<?php echo(strtotime("now") . "<br>"); echo(strtotime("3 October 2005") . "<br>"); echo(strtotime("+5 hours") . "<br>"); echo(strtotime("+1 week") . "<br>"); echo(strtotime("+1 week 3 days 7 hours 5 seconds") . "<br>"); echo(strtotime("next Monday") . "<br>"); echo(strtotime("last Sunday"));?>So your Final Well form code is
$expire_datetime = date('g:ia \o\n l jS F Y',strtotime($row->created, "+2 weeks"));
strtotime() function requires relative date format in first argument. Second argument is optional and can be unix time stamp.