The history of execution PHP code is based on internal multiple steps. PHP, which is a well-known server-side scripting language for building websites with simple and efficient code, executes code through the specified, layered system. In this paper, there will be a brief description of its significant steps and fragments as well as the main role of each of them in the overall performance.

1. PHP architecture in addition to principal constituents

Zend Engine is the heart of PHP and has a well-developed architecture of its own. Among the tasks implemented by this engine, one should mention the process of compilation of PHP code into the executable bytecode, as it is essential for further high-speed performance at the stage of runtime. The Zend Engine is a useful tool to improve your knowledge of performance enhancement methodologies.

Subtopics:

• Zend Engine: Coden adjustment is provided by the Zend Engine at the foundation of PHP, which is contacted directly and influences PHP’s operation. This engine, when understood, enables developers to set up applications in the right manner most efficiently.
• PHP Interpreter: PHP works on the principle of interpreted language, where each statement is converted for its execution and hence is a bit on the slower side while at the same time being very flexible.

2. How PHP Transforms Code: The Compilation Process

However, different from the compiled languages, PHP combines both compiling into binary codes and compilation into what is known as byte code. This two-stage compilation technique enables PHP to wed the interpretative characteristic of interpreted language with compiled language efficiency.

Steps Involved:

• Lexing and Parsing: The PHP interpreter then goes through the originally written syntax from top to bottom and produces tokens out of it. After this process of lexing, there is parsing, which sorts tokens into syntax trees.
• Bytecode Generation: The syntax tree to the PHP script is translated to the bytecode that is the low-level representation of the PHP script. The bytecode is now compiled for execution by the Zend Engine, which in turn makes the coding runtime faster and more efficient.

3. Execution Flow: How PHP Runs Code

In PHP, code execution is linear, and the compiling is carried out as well as the executing in cycles per line of a script. Yet, to enhance this flow, PHP uses caching such as opcode caching, which is the set of compiled byte codes that are necessary to run the matter faster in following requests.

Components of Execution Flow:

• Opcode Caching: OPcache helps to optimize PHP execution flow by storing precompiled code. If opcode caching is used, it becomes possible to work with the script loading time that has been reduced by several times.
• Garbage Collection: Most of PHP’s memory management is based on garbage collection, a form of memory recycling necessary for effective handling of big programs.

4. Optimization of Opcode in PHP

Opcode, or operational code, is the compiled version of PHP code. This bytecode enables a second fast performance once it is generated because code does not need to be compiled repeatedly.

Optimizing with Opcode:

• The application of opcode caching can help to decrease those parameters we’re speaking of as execution times. This is especially useful in those conditions when the code runs frequently, such as, for example, in the case of high traffic.
• Memory Optimization: Opcode caching helps in minimizing servers’ memory utilization by deleting the need for the recompiling of scripts each time requests are made.

5. PHP Runtime Extensions

One of the main sources of PHP’s flexibility is, without doubt, its variety of extensions. These include database drivers, encryption libraries, and image manipulation tools that plug into the runtime of PHP as opposed to the web server and improve functionality without sacing performance.

Common Extensions:

• MySQLi and PDO: These extensions are very important in handling database interactions through simplifying database connectivity and functionality.
• cURL: Provides transfer of data with the remote server; it supports integration of the RESTful services.
• GD and ImageMagick: As for digital imaging, which is a significant and essential part of the Web, where dynamic image manipulation is sometimes necessary.

6. PHP and Memory Management

Memory management is one of the core aspects of what PHP does in the course of its operation. Knowledge of methods of PDO PHP memory management is rather beneficial when improving large applications and preventing memory pitfalls.

Key Points: 

• Reference Counting: Like most scripting languages, PHP ‘uses the'reference counting’ method of garbage collection, put into practice to automatically check the number of reference links pointing to a variable that has been assigned memory and release the memory if no reference is found. 
• Garbage Collection: The PHP garbage collector frees up unused memory for long-running applications that include a web server.

7. PHP Internals Debugging of Performance

PHP optimization often involves internal level debugging when it will decide bottlenecks either within the execution flow or in memory usage.

Tools and Techniques:

• Profiling: It makes use of Xdebug, among many other tools, to analyze the resources being used and where you are consuming them highly.
• Logging: PHP offers its developers error handling functions and external tools that can be applied to monitor performance and debug in real time.

8. The Future of PHP Execution and Internals

The language is updated with the new releases of PHP 8 and other versions, and a few new features included within this are Just-In-Time (JIT) compilation; these compile parts of the code at run-time. This might help fill in some potential performance gap between interpreted languages and completely compiled ones.

Looking ahead:

• JIT Compilation: JIT unlocks new frontiers in performance for computations
• Better Memory Management: Optimizations to garbage collection and memory are having a strengthening effect on the robustness of PHP code for larger applications.
• Asynchronous Programming: Asynchronous execution support has the potential, which would be optimized within the coming editions of PHP, thereby unlocking a new array of use cases for the real-time applications built on PHP.

Inside the knowledge gained by analyzing PHP's internals is information that will provide the most profound insight for optimized speed, efficiency, and scalability in an application. As the landscape of PHP continues to evolve, knowledge of tools such as the Zend Engine and opcode caching will remain definitive in achieving high-performance applications.