refactor: upgrade to PHP 5.6

src/admin/layout/class-init.php

@@ -165,6 +165,7 @@ class Init {
 	 * @since 1.5.0
 	 */
 	public function get_plugin_meta_information(): void {
+		$l_str_plugin_name = null;
 		// TODO: add nonce verification?
 
 		// Get plugin internal name from POST data.

src/public/class-parser.php

@@ -1706,6 +1706,7 @@ class Parser {
 	 */
 	public function reference_container(): string {
 
+		$l_str_use_backbutton_hint = null;
 		// No footnotes have been replaced on this page.
 		if ( empty( self::$a_arr_footnotes ) ) {
 			return '';