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add_filter( 'register_post_type_args', 'portfolio_args', 10, 2 );
function portfolio_args( $args, $post_type ) {
	if ( 'la_portfolio' === $post_type ) {
		$args['rewrite']['slug'] = 'collectie';
		$args['label'] = 'Collecties';
		$args['name'] = 'Collecties';
		$args['singular_name'] = 'Collectie';
	}

	return $args;
}