wp-includes/rest-api/class-wp-rest-request.php:264Appends a header value for the given header.
$keystring$valuestring public function add_header( $key, $value ) { $key = $this->canonicalize_header_name( $key ); $value = (array) $value; if ( ! isset( $this->headers[ $key ] ) ) { $this->headers[ $key ] = array(); } $this->headers[ $key ] = array_merge( $this->headers[ $key ], $value ); }Introduced in 4.4.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/rest-api/class-wp-rest-request.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.