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    <h1>=&amp;</h1> <p><code>$a =&amp; $b</code> turns <code>$a</code> into an alias for <code>$b</code>. If the value or reference of <code>$a</code> is changed, the value or reference of <code>$b</code> will change accordingly.</p> <p>This differs from "both pointing to the same place" when it comes to objects: I could do <code>$c = $d = new AnObject(</code>), and both variables would point to the same place; however, changing where one points would not change where the other points. That is, <code>$c = null</code> would not make <code>$d = null</code>. In the case of <code>$a =&amp; $b</code>, however, <code>$a = null</code> would make <code>$b = null</code>.</p> <p><em>Note: Officially, aliases are actually called references. The official terminology is a bit of a misnomer and is certainly ambiguous, so I've opted to use the term "alias" instead. For documentation, see <a href="http://php.net/manual/en/language.references.php" rel="noreferrer">php.net</a>.</em></p> <h2>Uses and effects</h2> <p>With scalar values, <code>=&amp;</code> is sort of like wrapping the value in an object, so that you can change the value universally among several variables. With types that are normally passed by reference (objects), <code>=&amp;</code> provides a reference to a reference.</p> <p>I tend to use <code>=&amp;</code> when I'm working with associative arrays. Rather than rewriting <code>$foo['bar']['foobar']</code> several times over, I can create an alias: <code>$foobar =&amp; $foo['bar']['foobar']</code>. These even works if the index doesn't exist yet. If <code>$foo['bar']['foobar']</code> doesn't exist, then <code>isset($foobar)</code> will be false. It's better than using a plain old variable, because I can create the alias before testing for the existence of the key without triggering an error.</p> <p>Just be sure to unset (<code>unset($foobar)</code>) the alias when you're done. Otherwise, if you reuse the variable name later, you'll end up overwriting whatever the alias was pointing to.</p> <p>You can use aliases in other ways, too--they're not limited to assignments. They work with:</p> <ul> <li>foreach loops: <code>foreach ($a as &amp;$b)</code> Assigning to <code>$b</code> will overwrite the corresponding value in <code>$a</code>. Unset <code>$b</code> when you're done, or you'll run into weird problems!</li> <li>function/method parameters: <code>function foobar(&amp;$a)</code> Assigning to <code>$a</code> within <code>foobar</code> will change whatever variable the caller passed as <code>$a</code>.</li> <li>function/method return values: <code>function &amp;foobar()</code> Whatever is returned can be modified by the caller; this is useful for passing around aliases. It's also easy to abuse.</li> <li>arrays: <code>$a = array(&amp;$b)</code> Any changes to <code>$a[0]</code> will now affect <code>$b</code>, including assignments.</li> <li>call_user_func_array: <code>call_user_func('foobar', array(&amp;$a))</code> Assuming <code>foobar</code> takes a single alias parameter, <code>foobar</code> can now modify <code>$a</code>. This allows you to call functions/methods with alias parameters using <code>call_user_func_array</code>.</li> </ul> <h2>Examples</h2> <h3>Scalars</h3> <pre><code>$original = 1; $copy = $original; $reference =&amp; $original; // All three variables == 1. $reference = 2; // $original == 2, $reference == 2, $copy == 1 $original = 3; // $original == 3, $reference == 3, $copy == 1 $copy = 4; // $original == 3, $reference == 3, $copy == 4 </code></pre> <h3>Objects</h3> <pre><code>#!/usr/bin/env php &lt;?php class Object { private $properties; public function __construct(array $properties = array()) { $this-&gt;properties = $properties; } public function __isset($key) { return isset($this-&gt;properties[$key]); } public function __unset($key) { unset($this-&gt;properties[$key]); } public function __get($key) { return isset($this-&gt;$key) ? $this-&gt;properties[$key] : null; } public function __set($key, $value) { $this-&gt;properties[$key] = $value; } public function __toString() { return print_r($this-&gt;properties, true); } } function print_vars() { global $original, $ref, $refref; echo '$original: ', $original, '$ref: ', $ref, '$refref: ', $refref, PHP_EOL; } $original = new Object(array('a' =&gt; 1, 'b' =&gt; 2, 'c' =&gt; 3)); $ref = $original; $refref =&amp; $original; print_vars(); /* $original: Array ( [a] =&gt; 1 [b] =&gt; 2 [c] =&gt; 3 ) $ref: Array ( [a] =&gt; 1 [b] =&gt; 2 [c] =&gt; 3 ) $refref: Array ( [a] =&gt; 1 [b] =&gt; 2 [c] =&gt; 3 ) */ $original-&gt;a = 'duck'; $ref-&gt;b = 'moose'; $refref-&gt;c = 'cow'; print_vars(); /* $original: Array ( [a] =&gt; duck [b] =&gt; moose [c] =&gt; cow ) $ref: Array ( [a] =&gt; duck [b] =&gt; moose [c] =&gt; cow ) $refref: Array ( [a] =&gt; duck [b] =&gt; moose [c] =&gt; cow ) */ // This carries over to $refref, but not $ref. $original = new Object(array('x' =&gt; 1, 'y' =&gt; 2, 'z' =&gt; 3)); print_vars(); /* $original: Array ( [x] =&gt; 1 [y] =&gt; 2 [z] =&gt; 3 ) $ref: Array ( [a] =&gt; duck [b] =&gt; moose [c] =&gt; cow ) $refref: Array ( [x] =&gt; 1 [y] =&gt; 2 [z] =&gt; 3 ) */ // This does *not* carry over to $original or $ref. $ref = new Object(array('o' =&gt; 42, 'm' =&gt; 123, 'n' =&gt; 1337)); print_vars(); /* $original: Array ( [x] =&gt; 1 [y] =&gt; 2 [z] =&gt; 3 ) $ref: Array ( [o] =&gt; 42 [m] =&gt; 123 [n] =&gt; 1337 ) $refref: Array ( [x] =&gt; 1 [y] =&gt; 2 [z] =&gt; 3 ) */ // This *does* carry over to $original, but not $ref. $refref = new Object(array('alpha' =&gt; 10, 'beta' =&gt; 20, 'gamma' =&gt; 30)); print_vars(); /* $original: Array ( [alpha] =&gt; 10 [beta] =&gt; 20 [gamma] =&gt; 30 ) $ref: Array ( [o] =&gt; 42 [m] =&gt; 123 [n] =&gt; 1337 ) $refref: Array ( [alpha] =&gt; 10 [beta] =&gt; 20 [gamma] =&gt; 30 ) */ ?&gt; </code></pre> <h1>&amp;=</h1> <p><code>&amp;=</code> is unrelated to <code>=&amp;</code>. It comes from a set of assignment operations. Here's just a few:</p> <ul> <li><code>+=</code></li> <li><code>-=</code></li> <li><code>*=</code></li> <li><code>/=</code></li> </ul> <p>See the trend here?</p> <p>Binary arithmetic operators generally have assignment counterparts. Let's say <code>@</code> were an arithmetic operator (it's not as of writing) such that <code>$a @ $b</code> generally yields a number when <code>$a</code> and <code>$b</code> are numbers. (Think: Addition, multiplication, division, etc.) How often do you need to do something like this?</p> <pre><code>$a = $a @ $b; </code></pre> <p>Pretty often. Doesn't it seem a bit unnecessary to repeat <code>$a</code>? Many languages, including PHP, solve this with an array of assignment operators:</p> <pre><code>$a @= $b; </code></pre> <p>Much simpler, and to a programmer accustomed to this notation, perhaps more concise and descriptive at a glance. (I certainly find it easier to read, since I'm so used to it.) So to double a variable:</p> <pre><code>$a *= 2; </code></pre> <p>Quick, easy, and relatively descriptive. Some languages, including PHP, extend this feature beyond arithmetic for an extra operation or two. Notably:</p> <pre><code>$a = $a . 'Appended text'; // Is the same as: $a .= 'Appended text'; </code></pre> <p>Very useful.</p> <p><code>&amp;=</code> falls among these assignment operators, because <code>&amp;</code> represents a <a href="http://php.net/manual/en/language.operators.bitwise.php" rel="noreferrer">bitwise arithmetic AND operation</a>. There are a few others listed in the PHP documentation (see aforementioned link), all of which are common to many programming languages.</p> <p>This means that <code>$a &amp;= $b</code> is the same as <code>$a = $a &amp; $b</code>.</p>
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