Originally Posted by gornyhuy

In case anyone is keyword searching, there actually is a backward-INcompatible change from PHP4.X to php5.X involving arrays.

In php5 If you don't explicitly declare a variable as an array and then later you try to use certain array functions on it (slice, splice, etc), it will not work.

I learned this the hard way when I upgraded a dedicated box with a bunch of legacy php code on it.