I managed to solve it myself so thought I would post my solution for anyone else who might like to do this;

I created a file called datediff.php which contained the following;

php:
``<?php    function datediff(\$start_date,\$end_date,\$unit="y")        {            \$unit = strtoupper(\$unit);            \$start=strtotime(\$start_date);            if (\$start === -1) {                print("invalid start date");            }                        \$end=strtotime(\$end_date);                        if (\$end === -1) {                print("invalid end date");            }                        if (\$start > \$end) {                \$temp = \$start;                \$start = \$end;                \$end = \$temp;            }                        \$diff = \$end-\$start;                        \$day1 = date("j", \$start);            \$mon1 = date("n", \$start);            \$year1 = date("Y", \$start);            \$day2 = date("j", \$end);            \$mon2 = date("n", \$end);            \$year2 = date("Y", \$end);                        switch(\$unit) {                case "D":                    print(intval(\$diff/(24*60*60)));                    break;                case "M":                    if(\$day1>\$day2) {                        \$mdiff = ((\$year2-\$year1)*12)+(\$mon2-\$mon1-1);                    } else {                        \$mdiff = ((\$year2-\$year1)*12)+(\$mon2-\$mon1);                    }                    print(\$mdiff);                    break;                case "Y":                    if((\$mon1>\$mon2) || ((\$mon1==\$mon2) && (\$day1>\$day2))){                        \$ydiff = \$year2-\$year1-1;                    } else {                        \$ydiff = \$year2-\$year1;                    }                    print(\$ydiff);                    break;                case "YM":                    if(\$day1>\$day2) {                        if(\$mon1>=\$mon2) {                            \$ymdiff = 12+(\$mon2-\$mon1-1);                        } else {                            \$ymdiff = \$mon2-\$mon1-1;                        }                    } else {                        if(\$mon1>\$mon2) {                            \$ymdiff = 12+(\$mon2-\$mon1);                        } else {                            \$ymdiff = \$mon2-\$mon1;                        }                    }                    print(\$ymdiff);                    break;                case "YD":                    if((\$mon1>\$mon2) || ((\$mon1==\$mon2) &&(\$day1>\$day2))) {                        \$yddiff = intval((\$end - mktime(0, 0, 0, \$mon1, \$day1, \$year2-1))/(24*60*60));                                            } else {                        \$yddiff = intval((\$end - mktime(0, 0, 0, \$mon1, \$day1, \$year2))/(24*60*60));                    }                    print(\$yddiff);                    break;                case "MD":                    if(\$day1>\$day2) {                        \$mddiff = intval((\$end - mktime(0, 0, 0, \$mon2-1, \$day1, \$year2))/(24*60*60));                                            } else {                        \$mddiff = intval((\$end - mktime(0, 0, 0, \$mon2, \$day1, \$year2))/(24*60*60));                    }                    print(\$mddiff);                    break;                default:                 print("{Datedif Error: Unrecognized \\$unit parameter. Valid values are 'Y', 'M', 'D', 'YM'. Default is 'D'.}");                            }        }?>``

Then inlcuded it;

php:
``<?php include('dateDiff.php'); ?>``

Then added the code to my page;

php:
``<?php           \$date_1 = \$row_member_profile['dog_dob'];          \$date_2 = \$row_member_profile['years'];          datediff ("\$date_1","\$date_2","y"); ?>``