Why Does PHP Throw a 'Constant Expression Contains Invalid Operations' Error When Defining Static Properties?

PHP Error: Constant Expression Contains Invalid Operations

A common issue encountered in PHP development is the "Fatal error: Constant expression contains invalid operations" error, which occurs when you attempt to use an invalid operation within a constant expression. This typically occurs when defining class properties or function parameters as constants.

Problem

Specifically, in the provided example, the error is encountered on line 214 of the config.php file:

protected static $dbname = 'mydb_'.$appdata['id'];

This line attempts to define a static property named $dbname, initializing it with a concatenation of the string 'mydb_' and the value of the $appdata['id'] variable. However, this initialization is invalid because $appdata['id'] is not a constant expression.

Solution

The solution to this error is to ensure that all static property or parameter declarations are initialized with literal or constant values that can be evaluated at compile time. In this case, $appdata['id'] is not a constant value, so it cannot be used in the constant expression.

There are two possible ways to resolve this:

1. Remove the static attribute:

protected $dbname = 'mydb_'.$appdata['id'];

By removing the static attribute, the property becomes a dynamic property that is initialized at runtime when the variable $appdata['id'] is defined.

2. Use a constant string:

private static $dbname = 'mydb_' . 'CONSTANT_STRING';

By using a constant string, the initialization becomes a constant expression that can be evaluated at compile time.

Additional Information

It's important to understand that static declarations are resolved during compilation. This means that the values of variables and other dynamic expressions cannot be used in constant declarations.

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