What potential issues can arise when upgrading from PHP4 to PHP5.4, especially in terms of variable handling and syntax?

One potential issue when upgrading from PHP4 to PHP5.4 is the change in variable handling, specifically the introduction of the `$this` keyword in object-oriented programming. To solve this, ensure that all class methods within a class use `$this->` to access class properties and methods. 

class MyClass {
    private $myProperty;

    public function setProperty($value) {
        $this->myProperty = $value;
    }

    public function getProperty() {
        return $this->myProperty;
    }
}

Keywords

PHP4 PHP5.4 variable handling syntax backward compatibility

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