flatten.php

<?php

$_flatArray   = [];
$_nestedArray = [
	[
		'id'            => 1,
		'name'          => 'A',
		'subcategories' => [
			[
				'id'            => 2,
				'name'          => 'B',
				'subcategories' => []
			],
			[
				'id'            => 3,
				'name'          => 'C',
				'subcategories' => [
					[
						'id'            => 31,
						'name'          => 'CA',
						'subcategories' => []
					],
					[
						'id'            => 32,
						'name'          => 'CB',
						'subcategories' => [
							[
								'id'            => 321,
								'name'          => 'CBA',
								'subcategories' => []
							],
						]
					],
				]
			],
			[
				'id'            => 4,
				'name'          => 'D',
				'subcategories' => []
			],
			[
				'id'            => 5,
				'name'          => 'F',
				'subcategories' => []
			],
		]
	]
];

/**
 * @author Dennis Micky Jensen <dj@miinto.com>
 *
 * @param array $nestedArray
 * @param array $flattenedArray
 *
 * @return array
 */
function flattenArray(array $nestedArray, &$flattenedArray = [])
{
	foreach($nestedArray as $subcategory) {
		
		$_orphan = $subcategory;
		unset($_orphan['subcategories']);
		$flattenedArray[] = $_orphan;
		
		if(!empty($subcategory['subcategories'])) {
			flattenArray($subcategory['subcategories'], $flattenedArray);
		}
		
	}
	
	return $flattenedArray;
}

print '<pre>';
print_r(flattenArray($_nestedArray));

?>