Populating a PHP array: check for index first?

if (!isset($array[$key])) $array[$key] = $val;

$array[$key] = $val;

$anArray[ 'level1' ][ 'level2' ][ 'level3' ] = ...

$anArray[ 'level1' ][ 'level2' ]

<?php

function create_array_path( $path, & $inArray )
{
    if ( ! is_array( $inArray ) )
    {
    	throw new Exception( 'The second argument is not an array!' );
    }
    $traversed = array();
    $current = &$inArray;

    foreach( $path as $subpath )
    {
    	$traversed[] = $subpath;
    	if ( ! is_array( $current ) )
    	{
    		$current = array();
    	}
    	if ( ! array_key_exists( $subpath, $current ) )
    	{
    		$current[ $subpath ] = '';
    	}
    	$current = &$current[ $subpath ];
    }
}


$myArray = array();

create_array_path( array( 'level1', 'level2', 'level3' ), $myArray );

print_r( $myArray );

?>

Array
	(
	    [level1] => Array
	        (
	            [level2] => Array
	                (
	                    [level3] =>
	                )

	        )

	)

$array[$key] = !isset($array[$key]) ? $val : $array[$key];