Does PHP have threading?

I found this PECL package called threads, but there is not a release yet. And nothing is coming up on the PHP website.

From the PHP manual for the pthreads extension:

pthreads is an Object Orientated API that allows user-land multi-threading in PHP. It includes all the tools you need to create multi-threaded applications targeted at the Web or the Console. PHP applications can create, read, write, execute and synchronize with Threads, Workers and Stackables.

As unbelievable as this sounds, it's entirely true. Today, PHP can multi-thread for those wishing to try it.

The first release of PHP4, 22 May 2000, PHP was shipped with a thread safe architecture - a way for it to execute multiple instances of it's interpreter in separate threads in multi-threaded SAPI ( Server API ) environments. Over the last 13 years, the design of this architecture has been maintained and advanced: It has been in production use on the worlds largest websites ever since.

Threading in user land was never a concern for the PHP team, and it remains as such today. You should understand that in the world where PHP does it's business, there's already a defined method of scaling - add hardware. Over the many years PHP has existed, hardware has got cheaper and cheaper and so this became less and less of a concern for the PHP team. While it was getting cheaper, it also got much more powerful; today, our mobile phones and tablets have dual and quad core architectures and plenty of RAM to go with it, our desktops and servers commonly have 8 or 16 cores, 16 and 32 gigabytes of RAM, though we may not always be able to have two within budget and having two desktops is rarely useful for most of us.

Additionally, PHP was written for the non-programmer, it is many hobbyists native tongue. The reason PHP is so easily adopted is because it is an easy language to learn and write. The reason PHP is so reliable today is because of the vast amount of work that goes into it's design, and every single decision made by the PHP group. It's reliability and sheer greatness keep it in the spot light, after all these years; where it's rivals have fallen to time or pressure.

Multi-threaded programming is not easy for most, even with the most coherent and reliable API, there are different things to think about, and many misconceptions. The PHP group do not wish for user land multi-threading to be a core feature, it has never been given serious attention - and rightly so. PHP should not be complex, for everyone.

All things considered, there are still benefits to be had from allowing PHP to utilize it's production ready and tested features to allow a means of making the most out of what we have, when adding more isn't always an option, and for a lot of tasks is never really needed.

pthreads achieves, for those wishing to explore it, an API that does allow a user to multi-thread PHP applications. It's API is very much a work in progress, and designated a beta level of stability and completeness.

It is common knowledge that some of the libraries PHP uses are not thread safe, it should be clear to the programmer that pthreads cannot change this, and does not attempt to try. However, any library that is thread safe is useable, as in any other thread safe setup of the interpreter.

pthreads utilizes Posix Threads ( even in Windows ), what the programmer creates are real threads of execution, but for those threads to be useful, they must be aware of PHP - able to execute user code, share variables and allow a useful means of communication ( synchronization ). So every thread is created with an instance of the interpreter, but by design, it's interpreter is isolated from all other instances of the interpreter - just like multi-threaded Server API environments. pthreads attempts to bridge the gap in a sane and safe way. Many of the concerns of the programmer of threads in C just aren't there for the programmer of pthreads, by design, pthreads is copy on read and copy on write ( RAM is cheap ), so no two instances ever manipulate the same physical data, but they can both affect data in another thread. The fact that PHP may use thread unsafe features in it's core programming is entirely irrelevant, user threads, and it's operations are completely safe.

Why copy on read and copy on write:

public function run() {
    ...
    (1) $this->data = $data;
    ...
    (2) $this->other = someOperation($this->data);
    ...
}

(3) echo preg_match($pattern, $replace, $thread->data);

(1) While a read, and write lock are held on the pthreads object data store, data is copied from its original location in memory to the object store. pthreads does not adjust the refcount of the variable, Zend is able to free the original data if there are no further references to it.

(2) The argument to someOperation references the object store, the original data stored, which it itself a copy of the result of (1), is copied again for the engine into a zval container, while this occurs a read lock is held on the object store, the lock is released and the engine can execute the function. When the zval is created, it has a refcount of 0, enabling the engine to free the copy on completion of the operation, because no other references to it exist.

(3) The last argument to preg_match references the data store, a read lock is obtained, the data set in (1) is copied to a zval, again with a refcount of 0. The lock is released, The call to preg_match operates on a copy of data, that is itself a copy of the original data.

Things to know:

The object store's hash table where data is stored, thread safe, is
based on the TsHashTable shipped with PHP, by Zend.
The object store has a read and write lock, an additional access lock is provided for the TsHashTable such that if requires ( and it does, var_dump/print_r, direct access to properties as the PHP engine wants to reference them ) pthreads can manipulate the TsHashTable outside of the defined API.
The locks are only held while the copying operations occur, when the copies have been made the locks are released, in a sensible order.

This means:

When a write occurs, not only are a read and write lock held, but an additional access lock. The table itself is locked down, there is no possible way another context can lock, read, write or affect it.
When a read occurs, not only is the read lock held, but the additional access lock too, again the table is locked down.

No two contexts can physically nor concurrently access the same data from the object store, but writes made in any context with a reference will affect the data read in any context with a reference.

This is shared nothing architecture and the only way to exist is co-exist. Those a bit savvy will see that, there's a lot of copying going on here, and they will wonder if that is a good thing. Quite a lot of copying goes on within a dynamic runtime, that's the dynamics of a dynamic language. pthreads is implemented at the level of the object, because good control can be gained over one object, but methods - the code the programmer executes - have another context, free of locking and copies - the local method scope. The object scope in the case of a pthreads object should be treated as a way to share data among contexts, that is it's purpose. With this in mind you can adopt techniques to avoid locking the object store unless it's necessary, such as passing local scope variables to other methods in a threaded object rather than having them copy from the object store upon execution.

Most of the libraries and extensions available for PHP are thin wrappers around 3rd parties, PHP core functionality to a degree is the same thing. pthreads is not a thin wrapper around Posix Threads; it is a threading API based on Posix Threads. There is no point in implementing Threads in PHP that it's users do not understand or cannot use. There's no reason that a person with no knowledge of what a mutex is or does should not be able to take advantage of all that they have, both in terms of skill, and resources. An object functions like an object, but wherever two contexts would otherwise collide, pthreads provides stability and safety.

Anyone who has worked in java will see the similarities between a pthreads object and threading in java, those same people will have no doubt seen an error called ConcurrentModificationException - as it sounds an error raised by the java runtime if two threads write the same physical data concurrently. I understand why it exists, but it baffles me that with resources as cheap as they are, coupled with the fact the runtime is able to detect the concurrency at the exact and only time that safety could be achieved for the user, that it chooses to throw a possibly fatal error at runtime rather than manage the execution and access to the data.

No such stupid errors will be emitted by pthreads, the API is written to make threading as stable, and compatible as is possible, I believe.

Multi-threading isn't like using a new database, close attention should be paid to every word in the manual and examples shipped with pthreads.

Lastly, from the PHP manual:

pthreads was, and is, an experiment with pretty good results. Any of its limitations or features may change at any time; that is the nature of experimentation. It's limitations - often imposed by the implementation - exist for good reason; the aim of pthreads is to provide a useable solution to multi-tasking in PHP at any level. In the environment which pthreads executes, some restrictions and limitations are necessary in order to provide a stable environment.

Here is an example of what Wilco suggested:

$cmd = 'nohup nice -n 10 /usr/bin/php -c /path/to/php.ini -f /path/to/php/file.php action=generate var1_id=23 var2_id=35 gen_id=535 > /path/to/log/file.log & echo $!';
$pid = shell_exec($cmd);

Basically this executes the PHP script at the command line, but immediately returns the PID and then runs in the background. (The echo $! ensures nothing else is returned other than the PID.) This allows your PHP script to continue or quit if you want. When I have used this, I have redirected the user to another page, where every 5 to 60 seconds an AJAX call is made to check if the report is still running. (I have a table to store the gen_id and the user it's related to.) The check script runs the following:

exec('ps ' . $pid , $processState);
if (count($processState) < 2) {
     // less than 2 rows in the ps, therefore report is complete
}

There is a short post on this technique here: http://nsaunders.wordpress.com/2007/01/12/running-a-background-process-in-php/

There is nothing available that I'm aware of. The next best thing would be to simply have one script execute another via CLI, but that's a bit rudimentary. Depending on what you are trying to do and how complex it is, this may or may not be an option.

In short: yes, there is multithreading in php but you should use multiprocessing instead.

Backgroud info: threads vs. processes

There is always a bit confusion about the distinction of threads and processes, so i'll shortly describe both:

A thread is a sequence of commands that the CPU will process. The only data it consists of is a program counter. Each CPU core will only process one thread at a time but can switch between the execution of different ones via scheduling.
A process is a set of shared resources. That means it consists of a part of memory, variables, object instances, file handles, mutexes, database connections and so on. Each process also contains one or more threads. All threads of the same process share its resources, so you may use a variable in one thread that you created in another. If those threads are parts of two different processes, then they cannot access each others resources directly. In this case you need inter-process communication through e.g. pipes, files, sockets...

Multiprocessing

You can achieve parallel computing by creating new processes (that also contain a new thread) with php. If your threads do not need much communication or synchronization, this is your choice, since the processes are isolated and cannot interfere with each other's work. Even if one crashes, that doesn't concern the others. If you do need much communication, you should read on at "multithreading" or - sadly - consider using another programming language, because inter-process communication and synchronization introduces a lot of complexion.

In php you have two ways to create a new process:

let the OS do it for you: you can tell your operation system to create a new process and run a new (or the same) php script in it.

for linux you can use the following or consider Darryl Hein's answer:

$cmd = 'nice php script.php 2>&1 & echo $!';
pclose(popen($cmd, 'r'));

for windows you may use this:

$cmd = 'start "processname" /MIN /belownormal cmd /c "script.php 2>&1"';
pclose(popen($cmd, 'r'));

do it yourself with a fork: php also provides the possibility to use forking through the function pcntl_fork(). A good tutorial on how to do this can be found here but i strongly recommend not to use it, since fork is a crime against humanity and especially against oop.

Multithreading

With multithreading all your threads share their resources so you can easily communicate between and synchronize them without a lot of overhead. On the other side you have to know what you are doing, since race conditions and deadlocks are easy to produce but very difficult to debug.

Standard php does not provide any multithreading but there is an (experimental) extension that actually does - pthreads. Its api documentation even made it into php.net. With it you can do some stuff as you can in real programming languages :-) like this:

class MyThread extends Thread {
    public function run(){
        //do something time consuming
    }
}

$t = new MyThread();
if($t->start()){
    while($t->isRunning()){
        echo ".";
        usleep(100);
    }
    $t->join();
}

For linux there is an installation guide right here at stackoverflow's.

For windows there is one now:

First you need the thread-safe version of php.
You need the pre-compiled versions of both pthreads and its php extension. They can be downloaded here. Make sure that you download the version that is compatible with your php version.
Copy php_pthreads.dll (from the zip you just downloaded) into your php extension folder ([phpDirectory]/ext).
Copy pthreadVC2.dll into [phpDirectory] (the root folder - not the extension folder).
Edit [phpDirectory]/php.ini and insert the following line
```
extension=php_pthreads.dll
```
Test it with the script above with some sleep or something right there where the comment is.

And now the big BUT: Although this really works, php wasn't originally made for multithreading. There exists a thread-safe version of php and as of v5.4 it seems to be nearly bug-free but using php in a multi-threaded environment is still discouraged in the php manual (but maybe they just did not update their manual on this, yet). A much bigger problem might be that a lot of common extensions are not thread-safe. So you might get threads with this php extension but the functions you're depending on are still not thread-safe so you will probably encounter race conditions, deadlocks and so on in code you did not write yourself...

You can use pcntl_fork() to achieve something similar to threads. Technically it's separate processes, so the communication between the two is not as simple with threads, and I believe it will not work if PHP is called by apache.

Does PHP have threading?

Backgroud info: threads vs. processes

Multiprocessing

Multithreading

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