The variable members of a class are called "properties", or "fields" or "features", and are collectively referred to as "properties" in this document. A property declaration starts with the keywords public, protected or private, and is followed by an ordinary variable declaration.
Variables in attributes can be initialized, but the initialized value must be a constant. The constant here means that the PHP script can get its value during the compilation phase and does not rely on runtime information to evaluate.
It’s not that class variables cannot be assigned variables, but that this variable cannot be dynamic: such as the result of a method, such as a certain operation.
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The variable members of a class are called "properties", or "fields" or "features", and are collectively referred to as "properties" in this document. A property declaration starts with the keywords public, protected or private, and is followed by an ordinary variable declaration.
Variables in attributes can be initialized, but the initialized value must be a constant. The constant here means that the PHP script can get its value during the compilation phase and does not rely on runtime information to evaluate.
http://php.net/manual/zh/lang...
It’s not that class variables cannot be assigned variables, but that this variable cannot be dynamic: such as the result of a method, such as a certain operation.
Because this is the syntax of PHP...
The value in a class variable can only be a specific value, a number, a string or an array
It cannot be a class method
This is a grammatical rule, just remember it, there is no reason why