How is $this provided in PHP?

Question

I got a fundamental question about the $this keyword in PHP and how it is defined. Pracitcally, I know what it is and what it does. But I am interested in how it is provided / implemented?

For example, in C++ or Java, it is passed as a hidden parameter when calling non static member functions / methods. So from the compilers point of view, it is basically a parameter. Here is what the JVM specification says for example:

On instance method invocation, local variable 0 is always used to pass a reference to the object on which the instance method is being invoked (this in the Java programming language).

But how is it defined in PHP? AI chatbots keep telling me that it is associated with a "method context". Is that true? So when calling $this in PHP, it would not access a secretly passed parameter, but rather access the "associated context" of the method and get the object-handle from there? Is this "method context" acutally just an object that represents the method execution, where all the parameters are stored, together with other meta-data like the $this variable? Something like a more complex stack frame.

Or is it something totally different? If someone knows better, could you please elaborate a little bit on it?

• Greg Burghardt: Questions asking to explain code are off-topic for this community. As an aside, I thought a simple search would give me a definitive answer, but I'm not finding any official implementation notes. The PHP manual mentions "The pseudo-variable $this is available when a method is called from within an object context. $this is the value of the calling object." — and that's all I could find too.
• tweekz: It's not so much about code explanation, rather an "academic" question about the specification of the language. I also thought it would be easy to find, but neither php.net nor in the language specification talk a lot about it. I am just curious about such conceptual things we use everyday, but don't really know about in-depth.
• bdsl: It is effectively a parameter to the method, isn't it? It's just special because it's the one parameter that controls polymorphic function lookup, it doesn't have a user-controlled name, and its written to the left of the method name.
• Caleth: "in C++ it is passed as a hidden parameter" It is commonly implemented this way, but C++ does not define how it works, only what it does. Similarly you've read a reference from the JVM, not the Java language specification, about how that particular implementation of Java does it

Answer 1

As far as I can tell (in the current development version of Zend PHP, at least) it is stored on the call frame1:

struct _zend_execute_data {
 const zend_op *opline; /* executed opline */
 zend_execute_data *call; /* current call */
 zval *return_value;
 zend_function *func; /* executed function */
 zval This; /* this + call_info + num_args */
 zend_execute_data *prev_execute_data;
 zend_array *symbol_table;
 void **run_time_cache; /* cache op_array->run_time_cache */
 zend_array *extra_named_params;
};

When a method call is made, a frame is created, and the relevant object is stored in the This member of the struct.

Instances of $this in your code are compiled to the specialized opcode ZEND_FETCH_THIS, which calls zend_fetch_this_var when executed. The details are hidden behind macros, but I believe that (extremely simplified) this just sets the "result" value for the opcode to execute_data->This.

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1 A call frame is an item in the call stack. It keeps track of which function you're in, which instruction is currently being executed, assignment of local variables, and so on. One is created for every function call and it remains in use at least until the function returns.