Added explanatory comments to filter_input and filter_input_array to document

why some code that looks intuitively wrong is actually correct. Related to
bug #51344 (FILTER_NULL_ON_FAILURE flag automatically set in filter_input()
functions).

Author: LawnGnome

ext/filter/filter.c

@@ -780,6 +780,12 @@ PHP_FUNCTION(filter_input)
 				return;
 			}
 		}
+
+		/* The FILTER_NULL_ON_FAILURE flag inverts the usual return values of
+		 * the function: normally when validation fails false is returned, and
+		 * when the input value doesn't exist NULL is returned. With the flag
+		 * set, NULL and false should be returned, respectively. Ergo, although
+		 * the code below looks incorrect, it's actually right. */
 		if (filter_flags & FILTER_NULL_ON_FAILURE) {
 			RETURN_FALSE;
 		} else {
@@ -846,6 +852,12 @@ PHP_FUNCTION(filter_input_array)
 				PHP_FILTER_GET_LONG_OPT(option, filter_flags);
 			}
 		}
+
+		/* The FILTER_NULL_ON_FAILURE flag inverts the usual return values of
+		 * the function: normally when validation fails false is returned, and
+		 * when the input value doesn't exist NULL is returned. With the flag
+		 * set, NULL and false should be returned, respectively. Ergo, although
+		 * the code below looks incorrect, it's actually right. */
 		if (filter_flags & FILTER_NULL_ON_FAILURE) {
 			RETURN_FALSE;
 		} else {