Hàm group array trong php

Question

function array_group_by( array $array, $key )
{
	if ( ! is_string( $key ) && ! is_int( $key ) && ! is_float( $key ) && ! is_callable( $key ) ) {
		trigger_error( 'array_group_by(): The key should be a string, an integer, or a callback', E_USER_ERROR );
		return null;
	}

	$func = ( is_callable( $key ) ? $key : null );
	$_key = $key;

	// Load the new array, splitting by the target key
	$grouped = array();
	foreach ( $array as $value ) {
		if ( is_callable( $func ) ) {
			$key = call_user_func( $func, $value );
		}
		else {
			$key = $value[ $_key ];
		}

		$grouped[ $key ][] = $value;
	}

	// Recursively build a nested grouping if more parameters are supplied
	// Each grouped array value is grouped according to the next sequential key
	if ( func_num_args() > 2 ) {
		$args = func_get_args();

		foreach ( $grouped as $key => $value ) {
			$params = array_merge(array($value), array_slice( $args, 2, func_num_args() ) );
			$grouped[ $key ] = call_user_func_array( 'array_group_by', $params );
		}
	}

	return $grouped;
}