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The foreach construct provides an iterative mechanism specifically designed for traversing arrays and objects. It abstracts internal pointer management, automatically advancing through the iterable data structure from the first element to the last until exhaustion.

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Syntax

PHP supports two syntactical forms for the foreach loop: 1. Value-Only Iteration Assigns the value of the current element to the specified variable and advances the internal iteration state. 
foreach ($iterable as $value) {
    // Statement execution
}
2. Key-Value Iteration Assigns the current element’s key (index or string key) to $key and its value to $value. 
foreach ($iterable as $key => $value) {
    // Statement execution
}

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Technical Mechanics

Type Constraints The $iterable expression must evaluate to an array or an object. When an object is provided, it can either be a standard object (which iterates over its visible properties) or an object implementing the Traversable interface (such as Iterator or IteratorAggregate). As of PHP 8.0, attempting to iterate over an uniterable type (such as scalar types like integers, strings, and booleans, or the special type null) throws a TypeError. Pass-by-Value vs. Pass-by-Reference By default, foreach operates on a copy of the iterable’s values. Modifying the $value variable within the loop block does not mutate the original data structure. To mutate the original array elements directly, the value variable must be prefixed with the reference operator (&). 
$array = [1, 2, 3];

foreach ($array as &$value) {
    $value = $value * 2; // Mutates the original $array in memory
}
unset($value); // Destroys the dangling reference
Caveat: The reference to the last element remains in the current scope after the loop terminates. While the PHP engine does not enforce it as a strict technical requirement, calling unset($value) immediately after a by-reference foreach loop is a highly recommended best practice to prevent unintended mutations during subsequent variable assignments. Internal Pointer Behavior and Modification (PHP 7+) In PHP 7 and later, foreach does not utilize or modify the array’s internal pointer (which is manipulated by functions like current(), next(), or reset()). The behavior of the loop when the underlying array is modified depends on the iteration mode:
  • By-Value: foreach operates on a duplicated structure. If the array is modified (e.g., elements are appended or removed) within the loop, the loop continues to iterate over the original, unmodified copy.
  • By-Reference: foreach operates directly on the original array. If elements are appended to the array during a by-reference loop, the loop will iterate over the newly added elements.
Object Iteration When applied to a standard object, foreach iterates through all defined, non-static properties visible in the current execution scope. Iterating from outside the object exposes only public properties, whereas iterating from within an object’s own method exposes private and protected properties. Array Destructuring If the iterable contains nested arrays, foreach supports array destructuring directly within the loop declaration using the list() construct or the short array syntax []. 
$matrix = [[1, 2], [3, 4]];

foreach ($matrix as [$a, $b]) {
    // $a and $b represent the destructured elements of the nested arrays
}
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