Arrays in PHP treat integer and string integers synonymously. If you set $a['1'] = '1' and access $a[1] you will get '1'.
This is because php juggled your string key to an integer before assigning it; the assigment was actually made to an integer key.
PHP also juggles in the process of accesssing too which means if you try to access [1] with ['1'] it will work. Therefore
there is no real distinction between string integers and real integer, you can use them interchangably.
Remember that '1.0' does not equal '1' or 1 with strict comparison. This is true of array keys as well: you can have
both ['1.0'] and ['1'] and they would be different elements but not [1] and ['1'].