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copied!<p><a href="http://au2.php.net/isset" rel="nofollow noreferrer"><code>isset()</code></a> is the usual way of doing this:</p> <pre><code>if (!isset($blah)) { $blah = 'foo'; } </code></pre> <p><strong>Note:</strong> you can assign <code>null</code> to a variable and it will be assigned. This will yield different results with <code>isset()</code> and <a href="http://au2.php.net/manual/en/function.is-null.php" rel="nofollow noreferrer"><code>is_null()</code></a> so you need to be clear on what you mean by "not assigned". See <a href="http://brian.moonspot.net/null-isset-php" rel="nofollow noreferrer">Null vs. isset()</a>. This is also one case where you need to be careful about doing automatic type conversion, meaning using the <code>!=/==</code> or <code>===/!==</code> depending on the desired result.</p> <p>You can use boolean shorthand for this too (which is what the Perl <code>||=</code> operator is). As of PHP 5.2.x there is no operator like you speak of. In Perl:</p> <pre><code>$a ||= $b; </code></pre> <p>is equivalent to:</p> <pre><code>$a = $a || $b; </code></pre> <p>You can do the second form in PHP but PHP has some funky rules about <a href="http://us.php.net/manual/en/language.types.type-juggling.php" rel="nofollow noreferrer">type juggling</a>. See <a href="http://us.php.net/manual/en/language.types.boolean.php#language.types.boolean.casting" rel="nofollow noreferrer">Converting to boolean</a>:</p> <blockquote> <p>When converting to <a href="http://us.php.net/manual/en/language.types.boolean.php" rel="nofollow noreferrer">boolean</a>, the following values are considered FALSE:</p> <ul> <li>the <a href="http://us.php.net/manual/en/language.types.boolean.php" rel="nofollow noreferrer">boolean</a> <code>FALSE</code> itself</li> <li>the <a href="http://us.php.net/manual/en/language.types.integer.php" rel="nofollow noreferrer">integer</a> 0 (zero)</li> <li>the <a href="http://us.php.net/manual/en/language.types.float.php" rel="nofollow noreferrer">float</a> 0.0 (zero)</li> <li>the empty <a href="http://us.php.net/manual/en/language.types.string.php" rel="nofollow noreferrer">string</a>, and the <a href="http://us.php.net/manual/en/language.types.string.php" rel="nofollow noreferrer">string</a> "0"</li> <li>an <a href="http://us.php.net/manual/en/language.types.array.php" rel="nofollow noreferrer">array</a> with zero elements</li> <li>an <a href="http://us.php.net/manual/en/language.types.object.php" rel="nofollow noreferrer">object</a> with zero member variables (PHP 4 only)</li> <li>the special type <a href="http://us.php.net/manual/en/language.types.null.php" rel="nofollow noreferrer"><code>NULL</code></a> (including unset variables)</li> <li><a href="http://us.php.net/manual/en/ref.simplexml.php" rel="nofollow noreferrer">SimpleXML</a> objects created from empty tags</li> </ul> <p>Every other value is considered TRUE (including any resource).</p> </blockquote> <p>So after:</p> <pre><code>$a = 0; $a = $a || 5; </code></pre> <p>$a would equal 5. Likewise:</p> <pre><code>$a = 0; $b = ''; $c = $a == $b; // true $d = $a === $b; // false </code></pre> <p>You have to watch out for these things.</p>

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