Source

File: wp-includes/Requests/IDNAEncoder.php

public static function to_ascii($string) {
		// Step 1: Check if the string is already ASCII
		if (self::is_ascii($string)) {
			// Skip to step 7
			if (strlen($string) < 64) {
				return $string;
			}

			throw new Requests_Exception('Provided string is too long', 'idna.provided_too_long', $string);
		}

		// Step 2: nameprep
		$string = self::nameprep($string);

		// Step 3: UseSTD3ASCIIRules is false, continue
		// Step 4: Check if it's ASCII now
		if (self::is_ascii($string)) {
			// Skip to step 7
			if (strlen($string) < 64) {
				return $string;
			}

			throw new Requests_Exception('Prepared string is too long', 'idna.prepared_too_long', $string);
		}

		// Step 5: Check ACE prefix
		if (strpos($string, self::ACE_PREFIX) === 0) {
			throw new Requests_Exception('Provided string begins with ACE prefix', 'idna.provided_is_prefixed', $string);
		}

		// Step 6: Encode with Punycode
		$string = self::punycode_encode($string);

		// Step 7: Prepend ACE prefix
		$string = self::ACE_PREFIX . $string;

		// Step 8: Check size
		if (strlen($string) < 64) {
			return $string;
		}

		throw new Requests_Exception('Encoded string is too long', 'idna.encoded_too_long', $string);
	}