When to use $this->property instead of $property in PHP

Question

Super easy question. Look at the 2 sample class methods.

In the first One I pass in a variable/property call $params I then do $this->params

Answer 1

Use $this when accessing class variables.

When accessing a variable which is actually a parameter in a function, there's no need to utilize the $this keyword.. Actually, to access the function parameter named $params, you should not use the $this keyword...

In your example:

class TestClass{

    public function TestFunc($params){
       $this->params = $params;

       echo 'testing this something'. $this->params;
    }
}

$params from TestFunc($params){ is a parameter/argument of the function TestFunc and so you don't need to use $this. In fact, to access the parameter's value, you must not use $this -- Now when you used $this->params from $this->params = $params = $params;, you are actually setting a value equivalent to that of the parameter $params to a NEW class-level variable named also $params (since you didn't declare it anywhere in your sample code)

[edit] based on comment:

Look at this example:

class TestClass{

    public function TestFunc($params){
       $this->params = $params;
       # ^ you are setting a new class-level variable $params
       # with the value passed to the function TestFunc 
       # also named $params

       echo 'testing this something'. $this->params;
    }

    public function EchoParameterFromFunction_TestFunc() {
        echo "\n\$this->params: " . $this->params . "\n";
        # now you are echo-ing the class-level variable named $params
        # from which its value was taken from the parameter passed
        # to function TestFunc
    }

}

$tc = new TestClass();
$tc->EchoParameterFromFunction_TestFunc(); # error: undefined property TestClass::$params
$tc->TestFunc('TestFuncParam');
$tc->EchoParameterFromFunction_TestFunc(); # should echo: $this->params: TestFuncParam

The error when you called EchoParameterFromFunction_TestFunc without first calling TestFunc is a result of not declaring/setting the class-level variable/property named $params --you set this up inside TestFunc, which means it doesn't get set unless you call TestFunc. To set it right so that anyone can immediately access it is to:

class TestClass{
    # declare (and set if you like)
    public /*or private or protected*/ $params; // = ''; or create a construct...

    public function __construct(){
        # set (and first declare if you like)
        $this->params = 'default value';
    }
...
...
...

[edit : additional]

As @liquorvicar mentioned, which I also totally agree with is that you should always declare all your class-level properties/variables, regardless of whether or not you will use them. Reason being and as an example is that you don't want to access a variable that hasn't been set. See my example above which threw the error undefined property TestClass::$params..

Thanks to @liquorvicar for reminding me..