remove casts and ensure no out of bounds access

ext/standard/dl.c

@@ -135,8 +135,7 @@ PHPAPI int php_load_extension(char *filename, int type, int start_now)
 		}
 		libpath = estrdup(filename);
 	} else if (extension_dir && extension_dir[0]) {
-		int extension_dir_len = (int)strlen(extension_dir);
-		slash_suffix = IS_SLASH(extension_dir[extension_dir_len-1]);
+		slash_suffix = IS_SLASH(extension_dir[strlen(extension_dir)-1]);
 		/* Try as filename first */
 		if (slash_suffix) {
 			spprintf(&libpath, 0, "%s%s", extension_dir, filename); /* SAFE */

main/php_ini.c

@@ -353,7 +353,7 @@ static void php_load_php_extension_cb(void *arg)
 static void php_load_zend_extension_cb(void *arg)
 {
 	char *filename = *((char **) arg);
-	const int length = (int)strlen(filename);
+	const size_t length = strlen(filename);
 
 #ifndef PHP_WIN32
 	(void) length;
@@ -365,9 +365,13 @@ static void php_load_zend_extension_cb(void *arg)
 		DL_HANDLE handle;
 		char *libpath;
 		char *extension_dir = INI_STR("extension_dir");
-		int extension_dir_len = (int)strlen(extension_dir);
-		int slash_suffix = IS_SLASH(extension_dir[extension_dir_len-1]);
+		int slash_suffix = 0;
 		char *err1, *err2;
+
+		if (extension_dir && extension_dir[0]) {
+			slash_suffix = IS_SLASH(extension_dir[strlen(extension_dir)-1]);
+		}
+
 		/* Try as filename first */
 		if (slash_suffix) {
 			spprintf(&libpath, 0, "%s%s", extension_dir, filename); /* SAFE */