How Does PHP's `foreach` Loop Actually Work, and How Has Its Implementation Changed Between PHP 5 and PHP 7?

How foreach actually works in PHP?

Introduction:
foreach is used extensively in PHP for looping through arrays, objects, and other data structures. Understanding its inner workings is crucial to avoid unexpected behavior.

Three Types of foreach Iterables:
foreach supports three types of iterables:

• Arrays
• Normal objects
• Traversable objects

Traversable Object Iteration:
For Traversable objects, foreach acts as syntactical sugar for:

foreach ($it as $k => $v) {}

translates to:

if ($it instanceof IteratorAggregate) {
    $it = $it->getIterator();
}
for ($it->rewind(); $it->valid(); $it->next()) {
    $v = $it->current();
    $k = $it->key();
    // ...
}

Array and Object Iteration:

Ordered Dictionaries:
PHP arrays and objects are essentially ordered dictionaries that maintain the order of insertion. Iteration follows this order.

Conversion of Objects:
Packed object representations are converted to dictionaries during iteration. Plain-object iteration thus becomes similar to array iteration.

Internal Array Pointer (IAP):
Arrays in PHP 5 have an internal array pointer (IAP) that supports modifications and skipping over removed elements.

HashPointer:
foreach in PHP 5 uses a HashPointer to back up the IAP position and hash before running the loop body. It restores it afterward if the element still exists; otherwise, it uses the current IAP position.

Array Duplication:
foreach in PHP 5 duplicates arrays that are not references (is_ref=0) and have a refcount greater than 1.

PHP 5 Examples:

• In test cases 1 and 2, arrays are not duplicated because they have refcount=1 and are passed by value.
• In test case 3, the IAP is set to the end of the array after the loop due to element removal.
• In test cases 4 and 5, arrays are duplicated because the by-ref functions (each, reset) receive a reference to a duplicated array.

PHP 7 Changes:

Hashtable Iterators:
PHP 7 uses external hashtable iterators, registered in the array. Each iterator is advanced if the corresponding element is removed.

No IAP Usage by foreach:
foreach no longer uses the IAP, eliminating the need for duplication. By-value array iteration always uses the original elements.

Array Duplication:
Arrays are no longer duplicated during by-value iteration, regardless of their refcount or reference status.

PHP 7 Examples:

• In PHP 7, test cases 1 and 2 have the same output as in PHP 5 because the arrays are not duplicated.
• In PHP 7, test case 3 produces different output because the loop no longer affects the IAP.
• In PHP 7, test cases 4 and 5 behave similarly because each and reset still duplicate the array, while foreach uses the original array.

Additional PHP 7 Enhancements:

• Outer loops in nested iterations are no longer aborted when elements are removed in inner loops.
• The "arrow in time" problem is resolved.

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