Multi-threading is possible in php

Yes you can do multi-threading in PHP with pthreads

From the PHP documentation:

pthreads is an object-orientated API that provides all of the tools needed for multi-threading in PHP. PHP applications can create, read, write, execute and synchronize with Threads, Workers and Threaded objects.

Warning: The pthreads extension cannot be used in a web server environment. Threading in PHP should therefore remain to CLI-based applications only.

Simple Test

#!/usr/bin/php
<?php
class AsyncOperation extends Thread {

    public function __construct($arg) {
        $this->arg = $arg;
    }

    public function run() {
        if ($this->arg) {
            $sleep = mt_rand(1, 10);
            printf('%s: %s  -start -sleeps %d' . "\n", date("g:i:sa"), $this->arg, $sleep);
            sleep($sleep);
            printf('%s: %s  -finish' . "\n", date("g:i:sa"), $this->arg);
        }
    }
}

// Create a array
$stack = array();

//Initiate Multiple Thread
foreach ( range("A", "D") as $i ) {
    $stack[] = new AsyncOperation($i);
}

// Start The Threads
foreach ( $stack as $t ) {
    $t->start();
}

?>

First Run

12:00:06pm:     A  -start -sleeps 5
12:00:06pm:     B  -start -sleeps 3
12:00:06pm:     C  -start -sleeps 10
12:00:06pm:     D  -start -sleeps 2
12:00:08pm:     D  -finish
12:00:09pm:     B  -finish
12:00:11pm:     A  -finish
12:00:16pm:     C  -finish

Second Run

12:01:36pm:     A  -start -sleeps 6
12:01:36pm:     B  -start -sleeps 1
12:01:36pm:     C  -start -sleeps 2
12:01:36pm:     D  -start -sleeps 1
12:01:37pm:     B  -finish
12:01:37pm:     D  -finish
12:01:38pm:     C  -finish
12:01:42pm:     A  -finish

Real World Example

error_reporting(E_ALL);
class AsyncWebRequest extends Thread {
    public $url;
    public $data;

    public function __construct($url) {
        $this->url = $url;
    }

    public function run() {
        if (($url = $this->url)) {
            /*
             * If a large amount of data is being requested, you might want to
             * fsockopen and read using usleep in between reads
             */
            $this->data = file_get_contents($url);
        } else
            printf("Thread #%lu was not provided a URL\n", $this->getThreadId());
    }
}

$t = microtime(true);
$g = new AsyncWebRequest(sprintf("http://www.google.com/?q=%s", rand() * 10));
/* starting synchronization */
if ($g->start()) {
    printf("Request took %f seconds to start ", microtime(true) - $t);
    while ( $g->isRunning() ) {
        echo ".";
        usleep(100);
    }
    if ($g->join()) {
        printf(" and %f seconds to finish receiving %d bytes\n", microtime(true) - $t, strlen($g->data));
    } else
        printf(" and %f seconds to finish, request failed\n", microtime(true) - $t);
}

The stack is the memory set aside as scratch space for a thread of execution. When a function is called, a block is reserved on the top of the stack for local variables and some bookkeeping data. When that function returns, the block becomes unused and can be used the next time a function is called. The stack is always reserved in a LIFO (last in first out) order; the most recently reserved block is always the next block to be freed. This makes it really simple to keep track of the stack; freeing a block from the stack is nothing more than adjusting one pointer.

The heap is memory set aside for dynamic allocation. Unlike the stack, there's no enforced pattern to the allocation and deallocation of blocks from the heap; you can allocate a block at any time and free it at any time. This makes it much more complex to keep track of which parts of the heap are allocated or freed at any given time; there are many custom heap allocators available to tune heap performance for different usage patterns.

Each thread gets a stack, while there's typically only one heap for the application (although it isn't uncommon to have multiple heaps for different types of allocation).

To answer your questions directly:

To what extent are they controlled by the OS or language runtime?

The OS allocates the stack for each system-level thread when the thread is created. Typically the OS is called by the language runtime to allocate the heap for the application.

What is their scope?

The stack is attached to a thread, so when the thread exits the stack is reclaimed. The heap is typically allocated at application startup by the runtime, and is reclaimed when the application (technically process) exits.

What determines the size of each of them?

The size of the stack is set when a thread is created. The size of the heap is set on application startup, but can grow as space is needed (the allocator requests more memory from the operating system).

What makes one faster?

The stack is faster because the access pattern makes it trivial to allocate and deallocate memory from it (a pointer/integer is simply incremented or decremented), while the heap has much more complex bookkeeping involved in an allocation or deallocation. Also, each byte in the stack tends to be reused very frequently which means it tends to be mapped to the processor's cache, making it very fast. Another performance hit for the heap is that the heap, being mostly a global resource, typically has to be multi-threading safe, i.e. each allocation and deallocation needs to be - typically - synchronized with "all" other heap accesses in the program.

A clear demonstration:
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