How Does PHP's `foreach` Loop Work in PHP 5 and PHP 7?

Understanding PHP's ‘foreach’ Loop

‘foreach’ supports iterating over three types of values:

• Arrays
• Normal objects
• Traversable objects

Array and Object Iteration

Arrays and objects are traversed as ordered dictionaries. Traversing plain objects is similar to traversing arrays because they are internally represented as ordered dictionaries.

Internal Changes during Iteration

• By-reference array iteration or iteration of objects (by-handle semantics) allows in-loop modification while iterating.
• Iteration over arrays with reference count greater than 1 may duplicate the array before modification.

PHP 5 Approach

• Uses an internal array pointer (IAP) for tracking position.
• Additionally employs a HashPointer to support multiple simultaneous ‘foreach’ loops.
• Array duplication occurs if reference count is greater than 1 and the array is not a reference.
• IAP advancement occurs before the loop body runs.

PHP 7 Approach

• Abandons IAP usage entirely.
• Introduces hashtable iterators registered in the array to handle modifications mid-iteration.
• Array duplication in by-value array iteration occurs only if the array is modified during the loop.
• ‘foreach’ behavior is no longer influenced by ‘current()’ or ‘reset()’.

Answers to Your Questions

1. Is this correct and the whole story?

Your understanding of ‘foreach’ as working with an array copy but affecting the array pointer is correct. However, in PHP 7, ‘foreach’ no longer uses the IAP and instead relies on hashtable iterators, resulting in more consistent and predictable behavior.

2. What is it really doing?

• PHP 5: Uses a combination of IAP and HashPointer to support iterations, with potential for duplication and unexpected behavior in certain cases.
• PHP 7: Employs hashtable iterators that are unaffected by ‘foreach’ loop and properly handle modifications during iteration.

3. Are there any situations where using functions that adjust the array pointer during a ‘foreach’ can affect the outcome of the loop?

• PHP 5: ‘each()’ and ‘reset()’ can influence the behavior of nested ‘foreach’ loops and can even encounter unexpected behavior due to hash collisions.
• PHP 7: Functions that adjust the array pointer have no effect on ‘foreach’ behavior because it no longer relies on the IAP.

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