Creating an empty 2D array in PHP?

You don't create a 2d array in PHP, not in the traditional sense.

The suggestions above about $a = array(array()); in fact simply creating the following array:

$a = array(
    0 => array( )
);

Therefore, $a[0][0] = 'test'; would result in the following:

$a = array(
    0 => array(
        0 => 'test'
    )
);

At a first glance, it looks like it works, but in fact, this is still not a 2d array. When you try to use the 2nd row (index 1), at this point, PHP would throw a notice. Eg:

$a[1][0] = 'test2';

This is because $a[1] does not contain anything (remember that array(array()) simply creating an array at index 0?).

For it to work, you need to yet again do $a[1] = array();, or if you want to avoid indices you can do, $a[] = array();.

---

Example

$a = array(); // array of columns
for($c=0; $c<5; $c++){
    $a[$c] = array(); // array of cells for column $c
    for($r=0; $r<3; $r++){
        $a[$c][$r] = rand();
    }
}

The above code creates a 5x3 "2d array" of random numbers.

At its absolute simplest, a 2D dimensional array can be created as:

<?php
    $emptyArray = array(array());
?>

Or as of PHP 5.4 you can also use:

<?php
    $emptyArray = [[]];
?>