How to implement a read-only member variable in PHP?

Question

When trying to change it,throw an exception.

Answer 1

I suppose a solution, for class properties, would be to :

• not define a property with the name that interests you
• use the magic __get method to access that property, using the "fake" name
• define the __set method so it throws an exception when trying to set that property.
• See Overloading, for more informations on magic methods.

For variables, I don't think it's possible to have a read-only variable for which PHP will throw an exception when you're trying to write to it.

For instance, consider this little class :

class MyClass {
    protected $_data = array(
        'myVar' => 'test'
    );

    public function __get($name) {
        if (isset($this->_data[$name])) {
            return $this->_data[$name];
        } else {
            // non-existant property
            // => up to you to decide what to do
        }
    }

    public function __set($name, $value) {
        if ($name === 'myVar') {
            throw new Exception("not allowed : $name");
        } else {
            // => up to you to decide what to do
        }
    }
}

Instanciating the class and trying to read the property :

$a = new MyClass();
echo $a->myVar . '<br />';

Will get you the expected output :

test

While trying to write to the property :

$a->myVar = 10;

Will get you an Exception :

Exception: not allowed : myVar in /.../temp.php on line 19

Answer 2

class test {
   const CANT_CHANGE_ME = 1;
}

and you refer it as test::CANT_CHANGE_ME

Answer 3

Use a constant. Keyword const

Answer 4

이 효과를 얻는 속임수를 사용하는 것은 이렇게 할 수있는 방법이 없습니다.정상 열 (첫 번째가 추가됨)과 클러스터 된 열의 플롯을 추가하여 작동합니다 (하나의 시리즈는 모두 0 인 경우).모든 열의 너비는 동일하게 설정됩니다.

이제 내가 가지고있는 것입니다 :

function setupWeekElectricBar(Chart, theme, ClusteredColumns, Columns)
{
    var realData = [2.50,3.45,1.22,1.86,2.54, 4.01, {y:3.10, color: 'green'}]; // a new array
    var blankData = [0,0,0,0,0]; // a new array

    var targetData = [2.00,2.00,2.00,2.86,2.54, 2.00, 2.00];

    var chart = new Chart("weekElectricBar", {title: 'Daily Heating Cost',  titleGap: 0, titleFont: 'bold normal normal 15px Tahoma',  titleFontColor: "black"});

    chart.setTheme(theme);

     chart.addPlot("default", {
        type: ClusteredColumns,
        markers: true,
        gap: 5,
        minBarSize : 40,
        maxBarSize : 40,
        font: "bold normal 14px Tahoma",
        fontColor: "Black",
        shadows: {dx:4, dy:4}
    });

    chart.addPlot("back", {
        type: Columns,
        markers: true,
        gap: 5,
        minBarSize : 40,
        maxBarSize : 40,
        font: "bold normal 14px Tahoma",
        fontColor: "Black",
        shadows: {dx:4, dy:4}
    });

    chart.addAxis("y", {includeZero: true, fixUpper: 'major', vertical: true, min: 0, max:5, labels: [{value: 0, text: "£0"}, {value: 1, text: "£1"}, {value: 2, text: "£2"}, {value: 3, text: "£3"}, {value: 4, text: "£4"}, {value: 5, text: "£5"}] });
    chart.addAxis("x", {labels: [{value: 1, text: "Wed"}, {value: 2, text: "Thurs"}, {value: 3, text: "Fri"}, {value: 4, text: "Sat"}, {value: 5, text: "Sun"}, {value: 6, text: "Mon"}, {value: 7, text: "Last 24 Hrs"}]});

    chart.addSeries("blankData",blankData, {width: 2});
    chart.addSeries("realData",realData);
    chart.addSeries("targetData",targetData, {plot:'back'});

    chart.render();
}

.

Answer 5

The short answer is you can't create a read-only object member variable in PHP.

In fact, most object-oriented languages consider it poor form to expose member variables publicly anyway... (C# being the big, ugly exception with its property-constructs).

If you want a class variable, use the const keyword:

class MyClass {
    public const myVariable = 'x';
}

This variable can be accessed:

echo MyClass::myVariable;

This variable will exist in exactly one version regardless of how many different objects of type MyClass you create, and in most object-oriented scenarios it has little to no use.

If, however, you want a read-only variable that can have different values per object, you should use a private member variable and an accessor method (a k a getter):

class MyClass {
    private $myVariable;
    public function getMyVariable() {
        return $this->myVariable;
    }
    public function __construct($myVar) {
        $this->myVariable = $myVar;
    }
}

The variable is set in the constructor, and it's being made read-only by not having a setter. But each instance of MyClass can have its own value for myVariable.

$a = new MyClass(1);
$b = new MyClass(2);

echo $a->getMyVariable(); // 1
echo $b->getMyVariable(); // 2

$a->setMyVariable(3); // causes an error - the method doesn't exist
$a->myVariable = 3; // also error - the variable is private