How do you convert between 12 hour time and 24 hour time in PHP?

I have a time as a string, for example:

$time = '5:36 pm';

I require this to be in 24 hour format (i.e. 17:36), however I don't know which functions I need to use to convert it.

// 24-hour time to 12-hour time 
$time_in_12_hour_format  = date("g:i a", strtotime("13:30"));
output: 1:30 pm

// 12-hour time to 24-hour time 
$time_in_24_hour_format  = date("H:i", strtotime("1:30 PM"));
output: 13:30