BetterPHP

Hello!
I have an array that looks like this,

Array
(
    [1] => 400
    [2] => 900
    [3] => 1400
    [4] => 2100
    [5] => 2800
)

And I'm storing the current XP in a database along with the users level. Now I'm trying to find out a way to increment the users level by how much XP they have, not all that hard but if the user is level one and he gets 1200 XP directly, my system only increments the users level once per page-refresh and then it resets the XP to 0.

I asked this on stackoverflow and they told me not to reset the XP the only problem is if I don't do that then I have to completely redo the amount of XP per level, because when you get to level 2 you have 400 XP, and then you only need 500 XP to the next level, instead of 400 and then 920.

Any ideas on how I should do this? I currently have about 60 levels.