What are the ways in which php defines and uses constants

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Differential description

1. Const is a language structure, define () is a function, and const compiles faster than define ().

2. Define () declares constants. Constant names are string types, can be dynamically concatenated, and can be named with expressions. Const can only use ordinary constant names.

The constant defined by the const keyword is case-sensitive, and the define () function can determine whether it is case-sensitive by a third argument.

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