php - Use of undefined constant 0 - assumed ' 0' when using substr

Question

In php version 5.6, use the substr function to intercept strings. code show as below

substr("123456", 0, 2);

AppearUse of undefined constant 0 - assumed ' 0'

The online solution is to modify error_reporting in php.ini to turn off notice. But what is this essential question? ? If you don't close this notice configuration, how to solve it? ?