WP_REST_Server::send_header( string $key, string $value )

Source

File: wp-includes/rest-api/class-wp-rest-server.php

public function send_header( $key, $value ) {
		/*
		 * Sanitize as per RFC2616 (Section 4.2):
		 *
		 * Any LWS that occurs between field-content MAY be replaced with a
		 * single SP before interpreting the field value or forwarding the
		 * message downstream.
		 */
		$value = preg_replace( '/\s+/', ' ', $value );
		header( sprintf( '%s: %s', $key, $value ) );
	}