How to add elements to an empty array in PHP?

Both array_push and the method you described will work.

$cart = array();
$cart[] = 13;
$cart[] = 14;
// etc

//Above is correct. but below one is for further understanding
$cart = array();
    $cart[] = $i;  
echo "<pre>";
echo "</pre>";

Is the same as:

$cart = array();
array_push($cart, 13);
array_push($cart, 14);

// Or 
$cart = array();
array_push($cart, 13, 14);

Based on my experience, solution which is fine(the best) when keys are not important:

$cart = [];
$cart[] = 13;
$cart[] = "foo";
$cart[] = obj;

It's better to not use array_push and just use what you suggested. The functions just add overhead.

//We don't need to define the array, but in many cases it's the best solution.
$cart = array();

//Automatic new integer key higher than the highest 
//existing integer key in the array, starts at 0.
$cart[] = 13;
$cart[] = 'text';

//Numeric key
$cart[4] = $object;

//Text key (assoc)
$cart['key'] = 'test';

You can use array_push. It adds the elements to the end of the array, like in a stack.

You could have also done it like this:

$cart = array(13, "foo", $obj);