Source

File: wp-includes/formatting.php

function utf8_uri_encode( $utf8_string, $length = 0 ) {
	$unicode        = '';
	$values         = array();
	$num_octets     = 1;
	$unicode_length = 0;

	mbstring_binary_safe_encoding();
	$string_length = strlen( $utf8_string );
	reset_mbstring_encoding();

	for ( $i = 0; $i < $string_length; $i++ ) {

		$value = ord( $utf8_string[ $i ] );

		if ( $value < 128 ) {
			if ( $length && ( $unicode_length >= $length ) ) {
				break;
			}
			$unicode .= chr( $value );
			$unicode_length++;
		} else {
			if ( count( $values ) == 0 ) {
				if ( $value < 224 ) {
					$num_octets = 2;
				} elseif ( $value < 240 ) {
					$num_octets = 3;
				} else {
					$num_octets = 4;
				}
			}

			$values[] = $value;

			if ( $length && ( $unicode_length + ( $num_octets * 3 ) ) > $length ) {
				break;
			}
			if ( count( $values ) == $num_octets ) {
				for ( $j = 0; $j < $num_octets; $j++ ) {
					$unicode .= '%' . dechex( $values[ $j ] );
				}

				$unicode_length += $num_octets * 3;

				$values     = array();
				$num_octets = 1;
			}
		}
	}

	return $unicode;
}