opcache: Do not emit “temporary enabling” message when OPcache is already active

An easy way to accidentally enable OPcache “temporarily” is by using
`php_admin_value[opcache.enable]=1` within a FPM pool’s configuration, since
the `php_admin_value` settings mostly behave like settings in php.ini, with
many OPcache INI settings being a notable exception.

As long as OPcache is already enabled within php.ini (or simply by default),
emitting a warning for `php_admin_value[opcache.enable]=1` or similar is going
to be confusing, since is not actually temporarily enabling anything.

A follow-up commit will also try to detect this kind of incorrect configuration
and try to provide better advice for cases where OPcache is actually not yet
enabled.

Author: TimWolla

ext/opcache/tests/opcache_enable_noop_001.phpt

@@ -0,0 +1,24 @@
+--TEST--
+Dynamically setting opcache.enable does not warn when noop
+--INI--
+opcache.enable=1
+opcache.enable_cli=1
+--EXTENSIONS--
+opcache
+--FILE--
+<?php
+
+echo "Should not warn:", PHP_EOL;
+ini_set('opcache.enable', 1);
+echo "Disabling:", PHP_EOL;
+ini_set('opcache.enable', 0);
+echo "Should warn:", PHP_EOL;
+ini_set('opcache.enable', 1);
+
+?>
+--EXPECTF--
+Should not warn:
+Disabling:
+Should warn:
+
+Warning: Zend OPcache can't be temporary enabled (it may be only disabled till the end of request) in %s on line %d

ext/opcache/tests/opcache_enable_noop_002.phpt

@@ -0,0 +1,18 @@
+--TEST--
+Dynamically setting opcache.enable warns when not a noop
+--INI--
+opcache.enable=0
+opcache.enable_cli=1
+--EXTENSIONS--
+opcache
+--FILE--
+<?php
+
+echo "Should warn, since the INI was initialized to 0:", PHP_EOL;
+ini_set('opcache.enable', 1);
+
+?>
+--EXPECTF--
+Should warn, since the INI was initialized to 0:
+
+Warning: Zend OPcache can't be temporary enabled (it may be only disabled till the end of request) in %s on line %d

ext/opcache/zend_accelerator_module.c

@@ -162,6 +162,11 @@ static ZEND_INI_MH(OnEnable)
 		/* It may be only temporary disabled */
 		bool *p = (bool *) ZEND_INI_GET_ADDR();
 		if (zend_ini_parse_bool(new_value)) {
+			if (*p) {
+				/* Do not warn if OPcache is enabled, as the update would be a noop anyways. */
+				return SUCCESS;
+			}
+
 			zend_error(E_WARNING, ACCELERATOR_PRODUCT_NAME " can't be temporary enabled (it may be only disabled till the end of request)");
 			return FAILURE;
 		} else {