New event_is_declared() function

Note: use isset() instead of array_key_exists() as discussed in PR
#1534 (comment)

Replace usage of isset( $g_event_cache[...] ) elsewhere in event API,
by calls to the new function.

Fixes #25953

core/event_api.php

@@ -37,6 +37,17 @@
 # @global array $g_event_cache
 $g_event_cache = array();
 
+/**
+ * Determine if a given event has been declared
+ * @param string $p_name Event name
+ * @return bool True if the even has been declared
+ */
+function event_is_declared( $p_name ) {
+	global $g_event_cache;
+
+	return isset( $g_event_cache[$p_name] );
+}
+
 /**
  * Declare an event of a given type.
  * Will do nothing if event already exists.
@@ -48,7 +59,7 @@
 function event_declare( $p_name, $p_type = EVENT_TYPE_DEFAULT ) {
 	global $g_event_cache;
 
-	if( !isset( $g_event_cache[$p_name] ) ) {
+	if( !event_is_declared( $p_name ) ) {
 		$g_event_cache[$p_name] = array(
 			'type' => $p_type,
 			'callbacks' => array(),
@@ -80,7 +91,7 @@ function event_declare_many( array $p_events ) {
 function event_hook( $p_name, $p_callback, $p_plugin = 0 ) {
 	global $g_event_cache;
 
-	if( !isset( $g_event_cache[$p_name] ) ) {
+	if( !event_is_declared( $p_name ) ) {
 		error_parameters( $p_name );
 		trigger_error( ERROR_EVENT_UNDECLARED, WARNING );
 		return null;
@@ -135,7 +146,7 @@ function event_clear_callbacks() {
 function event_signal( $p_name, $p_params = null, $p_params_dynamic = null, $p_type = null ) {
 	global $g_event_cache;
 
-	if( !isset( $g_event_cache[$p_name] ) ) {
+	if( !event_is_declared( $p_name ) ) {
 		error_parameters( $p_name );
 		trigger_error( ERROR_EVENT_UNDECLARED, WARNING );
 		return null;