What’s Wrong

You’re trying to use a “variable variable” by building a variable name from two pieces, split and $i. This is not usually the best way to do things, and you’re doing it incorrectly.

$split($i) doesn’t mean “the variable named ‘split and the value of $i.'” It means, “take a variable named $split, treat its value as a function, and call that function with a parameter of $i.”

How to “Fix” It

If you really want to use variable variables, your syntax needs to be this:

${'split' . $i}

Again, this is a bad idea. You really should use an array instead.

How to Do It a Better Way

Use an array, as in

$split[1] = 1;
$split[2] = 2;
...

Then refer to the values as $split[$i] for whatever (valid) values of $i.