refactor: use native str_starts_with

Available from PHP 8. The future is now.

classes/form/context/render_context.php

@@ -146,7 +146,7 @@ abstract public function hide_if(string $dependant, string $dependency, string $
      * @return string name of the element qualified by this context's prefix
      */
     public function mangle_name(string $name): string {
-        if (utils::str_starts_with($name, $this->prefix)) {
+        if (str_starts_with($name, $this->prefix)) {
             // Already mangled, perhaps by an array_render_context.
             return $name;
         }

classes/question_ui_renderer.php

@@ -249,7 +249,7 @@ private function mangle_ids_and_names(): void {
                 ") as $attr
         ) {
             $original = $attr->value;
-            if ($attr->name === "usemap" && utils::str_starts_with($original, "#")) {
+            if ($attr->name === "usemap" && str_starts_with($original, "#")) {
                 // See https://developer.mozilla.org/en-US/docs/Web/API/HTMLImageElement/useMap.
                 $attr->value = "#" . $this->attempt->get_qt_field_name(substr($original, 1));
             } else {

classes/utils.php

@@ -42,18 +42,6 @@ public static function &ensure_exists(array &$array, string $key): array {
         return $array[$key];
     }
 
-    /**
-     * Determines whether a string starts with another.
-     *
-     * @param string $haystack
-     * @param string $needle
-     * @return bool
-     */
-    public static function str_starts_with(string $haystack, string $needle): bool {
-        // From https://stackoverflow.com/a/10473026.
-        return substr_compare($haystack, $needle, 0, strlen($needle)) === 0;
-    }
-
     /**
      * Given an array and a key such as `abc[def]`, returns `$array["abc"]["def"]`.
      *

question.php

@@ -160,11 +160,7 @@ public function apply_attempt_state(question_attempt_step $step) {
         $this->scoringstate = $this->get_behaviour()->get_qa()->get_last_qt_var(constants::QT_VAR_SCORING_STATE);
 
         $lastqtdata = $this->get_behaviour()->get_qa()->get_last_qt_data(null);
-        $lastresponse = $lastqtdata === null ? null : array_filter(
-            $lastqtdata,
-            fn($key) => !utils::str_starts_with($key, "_"),
-            ARRAY_FILTER_USE_KEY
-        );
+        $lastresponse = $lastqtdata === null ? null : utils::filter_for_response($lastqtdata);
 
         /* TODO: This method is also called from question_attempt->regrade and
                  question_attempt->start_question_based_on, where we shouldn't need to get the UI. */