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<?php $foo=''; $foo[]='test'; print_r($foo);
Output for 7.1.0 - 7.1.33, 7.2.0 - 7.2.33, 7.3.0 - 7.3.21, 7.4.0 - 7.4.9, 8.0.0alpha1 - beta1
Fatal error: Uncaught Error: [] operator not supported for strings in /in/AtO34:4 Stack trace: #0 {main} thrown in /in/AtO34 on line 4
Process exited with code 255.