Hmm. does PHP do string theory yet?
Anyway, having the numbers split to where they correspond to where humans split the hours of a day will mean that slightly more verbose conditionals will be needed. Working with "24 hour time" instead of 12 hour AM and PM, will make it a bit easier without a "noon" split, but you will still need to deal with the "midnight" split.
For example, you can't have a third shift as 2300 to 0700, it would need to be 2300 - 2400 and 0000 - 0700