Echoing array data prints only "Array"

I've been trying to solve Project Euler problem 1 and I feel I'm missing something fundamental. Would you please help me fix my bug?

I tried this first:

<?php
/* 
** Project Euler Problem 1
** If we list all the natural numbers below 10 that are multiples of 3 or 5,
** we get 3, 5, 6 and 9. The sum of these multiples is 23. 
** Find the sum of all the multiples of 3 or 5 below 1000.
*/
$numberOne = 3;
$numberTwo = 5;
$endingIndex = 1000;
$multiplesOfNumbers = array();  

for ($index = 1; $index <= $endingIndex; $index++) {
    if ($index % $numberOne == 0 && $index % $numberTwo == 0) {
        $multipleOfNumbers[] = $index;
    }
}
echo $multiplesOfNumbers;
?>

Output:

Array

So I tried to do it with array_push instead, like this:

<?php
/* 
** Project Euler Problem 1
** If we list all the natural numbers below 10 that are multiples of 3 or 5,
** we get 3, 5, 6 and 9. The sum of these multiples is 23. 
** Find the sum of all the multiples of 3 or 5 below 1000.
*/
$numberOne = 3;
$numberTwo = 5;
$endingIndex = 1000;
$multiplesOfNumbers = array();  

for ($index = 1; $index <= $endingIndex; $index++) {
    if ($index % $numberOne == 0 && $index % $numberTwo == 0) {
        // $multipleOfNumbers[] = $index;
        array_push($multiplesOfNumbers, $index);
    }
}
echo $multiplesOfNumbers;
?>

The output is the same. What am I missing?

Try this way:

print_r($multiplesOfNumbers);