The correct way to work with dates in PHP >= 5.2.0 is using DateTime, i.e.:
$startRaw = '2013-05-10 12:04:58';
$start = DateTime::createFromFormat('Y-m-d H:i:s', $startRaw );
$endRaw = date("2015-05-10 12:04:58");
$end = DateTime::createFromFormat('Y-m-d H:i:s', $endRaw );
$diff = $start->diff($end);
echo 'Difference: ' . $diff->format('%y years') . "\n";
Output:
Difference: 2 years
Demo:
http://ideone.com/3GzQsU
format
The following characters are recognized in the format parameter
  string. Each format character must be prefixed by a percent sign (%).
% Literal % % 
Y Years, numeric, at least 2 digits with leading 0 01, 03 
y Years, numeric 1, 3 
M Months, numeric, at least 2 digits with leading 0 01, 03, 12 
m Months, numeric 1, 3, 12 
D Days, numeric, at least 2 digits with leading 0 01, 03, 31 
d Days, numeric 1, 3, 31 
a Total amount of days 4, 18, 8123 
H Hours, numeric, at least 2 digits with leading 0 01, 03, 23 
h Hours, numeric 1, 3, 23 
I Minutes, numeric, at least 2 digits with leading 0 01, 03, 59 
i Minutes, numeric 1, 3, 59 
S Seconds, numeric, at least 2 digits with leading 0 01, 03, 57 
s Seconds, numeric 1, 3, 57 
R Sign "-" when negative, "+" when positive -, + 
r Sign "-" when negative, empty when positive -,  
Learn more about The DateTime class