How to push both value and key into PHP array

Nope, there is no array_push() equivalent for associative arrays because there is no way determine the next key.

You'll have to use

$arrayname[indexname] = $value;

Pushing a value into an array automatically creates a numeric key for it.

When adding a key-value pair to an array, you already have the key, you don't need one to be created for you. Pushing a key into an array doesn't make sense. You can only set the value of the specific key in the array.

// no key
array_push($array, $value);
// same as:
$array[] = $value;

// key already known
$array[$key] = $value;