add has_filter method

gump.class.php

@@ -1961,4 +1961,16 @@ public static function has_validator(string $rule): bool
     {
         return method_exists(__CLASS__, self::validator_to_method($rule)) || isset(self::$validation_methods[$rule]);
     }
+
+    /**
+     * Checks if a filter method exists for a given filter.
+     * @param string $filter
+     * @return bool
+     */
+    public static function has_filter(string $filter): bool
+    {
+        return method_exists(__CLASS__, self::filter_to_method($filter))
+            || isset(self::$filter_methods[$filter])
+            || function_exists($filter);
+    }
 }

tests/StaticHasFilterTest.php

@@ -0,0 +1,45 @@
+<?php
+
+namespace Tests;
+
+use GUMP;
+
+class StaticHasFilterTest extends BaseTestCase
+{
+    public function testHasFilterWhenExists(): void
+    {
+        $filterRules = [
+            // There are native filters
+            'noise_words',
+            'rmpunctuation',
+            'urlencode',
+            'htmlencode',
+            'sanitize_email',
+            'sanitize_numbers',
+            'sanitize_floats',
+            'sanitize_string',
+            'boolean',
+            'basic_tags',
+            'whole_number',
+            'ms_word_characters',
+            'lower_case',
+            'upper_case',
+            'slug',
+            // These are built-in functions
+            'trim',
+            'strtoupper',
+            'strtolower',
+            'intval',
+            'floatval',
+        ];
+
+        foreach ($filterRules as $filterRule) {
+            $this->assertTrue(GUMP::has_filter($filterRule));
+        }
+    }
+
+    public function testHasFilterWhenNotExists(): void
+    {
+        $this->assertFalse(GUMP::has_filter('custom_filter'));
+    }
+}