PHP two classes use the same namespace and cannot call each other

created two classes, C and D, both of which belong to the same namespace:func,. Instantiate C in the constructor of D and call the cc method in C, and class C under func cannot be found.

ClivedeMBP:Workspace clive$ php D.php 
PHP Fatal error:  Uncaught Error: Class "func\C" not found in /Users/clive/Workspace/D.php:7
Stack trace:
-sharp0 /Users/clive/Workspace/D.php(11): func\D->__construct()
-sharp1 {main}
  thrown in /Users/clive/Workspace/D.php on line 7

Fatal error: Uncaught Error: Class "func\C" not found in /Users/clive/Workspace/D.php on line 7

Error: Class "func\C" not found in /Users/clive/Workspace/D.php on line 7

Call Stack:
    0.0010     352608   1. {main}() /Users/clive/Workspace/D.php:0
    0.0029     352648   2. func\D->__construct() /Users/clive/Workspace/D.php:11

excuse me, how should two classes under the same namespace be written so that they can call each other?