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add_filter( 'woocommerce_currencies', 'wooninja_add_currencies' );

function wooninja_add_currencies( $currencies ) {
	$currencies['MNT'] = 'Mongolian Tughrik';
	return $currencies;
}

add_filter( 'woocommerce_currency_symbol', 'wooninja_currency_symbol', 10, 2 );

function wooninja_currency_symbol( $currency_symbol, $currency ) {
	if ( 'MNT' === $currency ) {
		$currency_symbol = '₮';
	}
	return $currency_symbol;
}