Source

File: wp-includes/class-http.php

public static function buildCookieHeader( &$r ) { // phpcs:ignore WordPress.NamingConventions.ValidFunctionName.MethodNameInvalid
		if ( ! empty( $r['cookies'] ) ) {
			// Upgrade any name => value cookie pairs to WP_HTTP_Cookie instances.
			foreach ( $r['cookies'] as $name => $value ) {
				if ( ! is_object( $value ) ) {
					$r['cookies'][ $name ] = new WP_Http_Cookie(
						array(
							'name'  => $name,
							'value' => $value,
						)
					);
				}
			}

			$cookies_header = '';
			foreach ( (array) $r['cookies'] as $cookie ) {
				$cookies_header .= $cookie->getHeaderValue() . '; ';
			}

			$cookies_header         = substr( $cookies_header, 0, -2 );
			$r['headers']['cookie'] = $cookies_header;
		}
	}