The problem is birthday is a string. You need to convert it into an integer first using strtotime
.
$rdiff = strtotime($a->birthday) - strtotime($b->birthday);
As those dates are in UK format you'll need to change the slashes between d/m/y to hyphens to, as when using slashes strtotime expects the date to be in US format, i.e. m/d/y. From the PHP manual:
Dates in the m/d/y or d-m-y formats are disambiguated by looking at the separator between the various components: if the separator is a slash (/), then the American m/d/y is assumed; whereas if the separator is a dash (-) or a dot (.), then the European d-m-y format is assumed.
So use:
$rdiff = strtotime(str_replace('/', '-', $a->birthday)) - strtotime(str_replace('/', '-', $b->birthday));