How to prevent repeated definition of constants in PHP

//To prevent repeated definition of constants, borrow
defined('YII_DEBUG') or define('YII_DEBUG',true);
defined('YII_TRACE_LEVEL') or define('YII_TRACE_LEVEL) in Yii ',3);
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?>

Instructions:

The or syntax is to execute the expression on the right when the expression on the left is flase. defined is to determine whether the constant is defined, and define is to define the constant. defined('YII_DEBUG') or define('YII_DEBUG',true); //Written like this, when the constant is defined as false, the constant is defined