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    <p><strong>A)</strong> To understand what's happening here, one needs to <a href="http://derickrethans.nl/files/phparch-php-variables-article.pdf" rel="nofollow noreferrer">understand PHP's handling of values/variables and references</a> (PDF, 1.2MB). As <a href="http://us.php.net/manual/en/language.references.whatare.php" rel="nofollow noreferrer">stated throughout the documentation</a>: <a href="http://us.php.net/manual/en/language.references.arent.php" rel="nofollow noreferrer">"references are not pointers"</a>; and <a href="http://us.php.net/manual/en/language.references.return.php" rel="nofollow noreferrer">you can only return variables by reference from a function</a> - nothing else.</p> <p>In my opinion, that means, any function in PHP will return a <strong>reference</strong>. But some functions (built in PHP) require <strong>values/variables</strong> as arguments. Now, if you are nesting function-calls, the inner one returns a reference, while the outer one expects a value. This leads to the 'famous' E_STRICT-error <a href="https://stackoverflow.com/search?q=only%20variables%20should%20be%20passed%20by%20reference">"Only variables should be passed by reference"</a>.</p> <pre><code>$fileName = 'example.txt'; $fileExtension = array_pop(explode('.', $fileName)); // will result in Error 2048: Only variables should be passed by reference in… </code></pre> <p><strong>B)</strong> I found a line in the <a href="http://www.icosaedro.it/articoli/php-syntax-ebnf.txt" rel="nofollow noreferrer">PHP-syntax description linked in the question</a>.</p> <pre><code>expr_without_variable = "(" expr ")" </code></pre> <p>In combination with this sentence from the <a href="http://us.php.net/manual/en/language.expressions.php" rel="nofollow noreferrer">documentation</a>: "In PHP, almost anything you write is an expression. The simplest yet most accurate way to define an expression is 'anything that has a value'.", this leads me to the conclusion that even <code>(5)</code> is an expression in PHP, which evaluates to an integer with the value 5.</p> <p>(As <code>$a = 5</code> is not only an assignment but also an expression, which evalutes to 5.)</p> <p><strong>Conclusion</strong> </p> <p>If you pass a reference to the expression <code>(...)</code>, this expression will return a value, which then may be passed as argument to the outer function. If that (my line of thought) is true, the following two lines should work equivalently:</p> <pre><code>// what I've used over years: (spaces only added for readability) $fileExtension = array_pop( ( explode('.', $fileName) ) ); // vs $fileExtension = array_pop( $tmp = explode('.', $fileName) ); </code></pre> <p>See also <a href="http://the-stickman.com/web-development/php/php-505-fatal-error-only-variables-can-be-passed-by-reference/" rel="nofollow noreferrer">PHP 5.0.5: Fatal error: Only variables can be passed by reference; 13.09.2005</a></p>
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    1. POParentheses altering semantics of function call result
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    1. CObut from this doc page: http://www.php.net/manual/en/language.references.pass.php it seems expressions cannot be used "as the result is undefined". I wonder if the whole parenthesis trick is not just bypassing internal checks, and may become in long term an undefined application result thing.
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    2. COWell this post is highly speculative. In the absence of a documentation (and I've searched for more than one hour, knowing how to use search engines), this is the best I can provide. My idea was, to commonly create a documentation for that behavior as a SO-wiki-entry.
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    3. COFWIW, `(5)` would be an expression in pretty much all C-like languages.
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