Fixed some PHP errors

src/php/ext/grpc/call_credentials.c

@@ -35,6 +35,7 @@
 
 #include <grpc/grpc.h>
 #include <grpc/grpc_security.h>
+#include <grpc/support/string_util.h>
 
 zend_class_entry *grpc_ce_call_credentials;
 #if PHP_MAJOR_VERSION >= 7

src/php/ext/grpc/call_credentials.h

@@ -65,9 +65,12 @@ typedef struct plugin_state {
 } plugin_state;
 
 /* Callback function for plugin creds API */
-void plugin_get_metadata(void *state, grpc_auth_metadata_context context,
-                         grpc_credentials_plugin_metadata_cb cb,
-                         void *user_data);
+int plugin_get_metadata(
+  void *ptr, grpc_auth_metadata_context context,
+  grpc_credentials_plugin_metadata_cb cb, void *user_data,
+  grpc_metadata creds_md[GRPC_METADATA_CREDENTIALS_PLUGIN_SYNC_MAX],
+  size_t *num_creds_md, grpc_status_code *status,
+  const char **error_details);
 
 /* Cleanup function for plugin creds API */
 void plugin_destroy_state(void *ptr);