What are the potential pitfalls of using outdated PHP syntax, such as the var keyword, in modern PHP development?

Using outdated PHP syntax, such as the var keyword, in modern PHP development can lead to compatibility issues with newer versions of PHP and may result in deprecated warnings or errors. To solve this issue, it is recommended to use the more modern and explicit data type declarations like public, protected, or private when defining class properties.

class MyClass {
    public $myProperty;
}