Question

$array = array (0.1 => 'a', 0.2 => 'b');
echo count ($array);

It overwrites first array element by second, just because, I used float with 0.

And hence output of above code is 1, instead of 2.

Why PHP round array index down to 0 ?

Answer 1

The array keys are interpreted as numeric, but numeric keys must be integers, Therefore, both float values are cast (truncated) to integer zero and 0.2 overwrites 0.1.

var_dump($array);
array(1) {
  [0]=>
  string(1) "b"
}

Make the array keys strings if you want to use non integer values:

$array = array ("0.1" => 'a', "0.2" => 'b');
echo count($array);
// 2

array(2) {
  ["0.1"]=>
  string(1) "a"
  ["0.2"]=>
  string(1) "b"
}