In-Depth Analysis of PHP7 Internal Mechanisms: Detailed Explanation of Function Calls and Variable Assignments

1. In-Depth Study of PHP7 Internal Development Principles

PHP, as a widely used programming language, has its underlying mechanisms extensively studied by developers. This article dives into the internal principles of PHP7, highlighting the implementation of function calls and variable assignments.

2. Function Call Mechanism

2.1 Call Stack

In PHP7, function calls are managed via a call stack. The call stack is a Last-In-First-Out (LIFO) data structure that stores relevant information for each function invocation, including function names and parameters. Once the function completes execution, its information is popped from the top of the stack.

Here is an example of a function call:

function foo($a, $b) {
   return $a + $b;
}
$result = foo(1, 2);

The call stack changes during execution as follows: initially empty, function foo’s info is pushed onto the stack when called, and popped off after completion with the result returned.

2.2 Actual Parameter Passing Mechanism

PHP7 supports various ways to pass arguments to functions:

• Pass by value: copies the argument’s value to the parameter; modifying the parameter inside the function does not affect the original argument.
• Pass by reference: passes the argument’s address; changes inside the function affect the original variable.
• Default parameters: if not passed, default values are used.
• Variable-length arguments: supports functions accepting variable numbers of arguments, passed as an array.

Example of pass-by-reference:

function foo(&$a) {
   $a = 2;
}
$b = 1;
foo($b);
echo $b; // Outputs 2

The function foo receives the address of $b, so modifying $a inside affects $b directly.

3. Variable Assignment Mechanism

PHP7 manages variable assignment using reference counting for memory management. Reference counting tracks how many references point to a variable; when it reaches zero, the memory is released.

3.1 Assignment Operation

During assignment, if a variable’s reference count is 1, the value is directly updated. If greater than 1, PHP creates a copy of the original value and assigns the new value, with the original memory reclaimed by the garbage collector.

Example code:

$a = 1;
$b = $a;
$c = &$a;
echo $a, $b, $c; // Outputs 111
$a = 2;
echo $a, $b, $c; // Outputs 212

Assigning $a to $b points both to the same data copy; $c is a reference to $a, so modifying $a also changes $c, while $b remains unchanged.

3.2 Garbage Collection Mechanism

Reference counting alone cannot handle circular references, which cause memory leaks. To address this, PHP7 introduces a mark-and-sweep garbage collection mechanism.

When a variable’s reference count reaches zero, it is marked as garbage. The garbage collector traverses the root set (globals and local variables), marking reachable objects. Unmarked objects are considered garbage and their memory is freed.

Example of circular reference:

class A {
   public $b;
}
class B {
   public $a;
}
$a = new A;
$b = new B;
$a->b = $b;
$b->a = $a;
unset($a, $b);

Here, $a and $b reference each other, preventing reference counts from reaching zero. After calling unset, the garbage collector detects the unreachable circular references and frees their memory.

4. Conclusion

Understanding PHP7’s internal principles, especially the function call and variable assignment mechanisms, is essential for improving development efficiency and optimizing performance. This article highlighted call stack management, parameter passing types, and reference counting combined with garbage collection, offering developers a comprehensive grasp of PHP7’s runtime behavior.