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    <p>As its name may suggest, <code>strip_tags</code> should remove all HTML tags. The only way we can proof it is by analyzing the source code. The next analysis applies to a <code>strip_tags('...')</code> call, without a second argument for whitelisted tags.</p> <p>First at all, some theory about HTML tags: a tag starts with a <code>&lt;</code> followed by non-whitespace characters. If this string starts with a <code>?</code>, it <a href="http://www.w3.org/TR/html4/conform.html#h-4.2" rel="noreferrer">should not be parsed</a>. If this string starts with a <code>!--</code>, it's considered a comment and the following text should neither be parsed. A comment is terminated with a <code>--&gt;</code>, inside such a comment, characters like <code>&lt;</code> and <code>&gt;</code> are allowed. Attributes can occur in tags, their values may optionally be surrounded by a quote character (<code>'</code> or <code>"</code>). If such a quote exist, it must be closed, otherwise if a <code>&gt;</code> is encountered, the tag is not closed.</p> <p>The code <code>&lt;a href="example&gt;xxx&lt;/a&gt;&lt;a href="second"&gt;text&lt;/a&gt;</code> is interpreted in Firefox as:</p> <pre><code>&lt;a href="http://example.com%3Exxx%3C/a%3E%3Ca%20href=" second"=""&gt;text&lt;/a&gt; </code></pre> <p>The PHP function <a href="http://php.net/strip-tags" rel="noreferrer"><code>strip_tags</code></a> is referenced in <a href="http://lxr.php.net/opengrok/xref/PHP_5_3/ext/standard/string.c#4036" rel="noreferrer">line 4036 of ext/standard/string.c</a>. That function calls the <a href="http://lxr.php.net/opengrok/xref/PHP_5_3/ext/standard/string.c#php_strip_tags_ex" rel="noreferrer">internal function php_strip_tags_ex</a>.</p> <p>Two buffers exist, one for the output, the other for "inside HTML tags". A counter named <code>depth</code> holds the number of open angle brackets (<code>&lt;</code>).<br> The variable <code>in_q</code> contains the quote character (<code>'</code> or <code>"</code>) if any, and <code>0</code> otherwise. The last character is stored in the variable <code>lc</code>.</p> <p>The functions holds five states, three are mentioned in the description above the function. Based on this information and the function body, the following states can be derived:</p> <ul> <li>State 0 is the output state (not in any tag)</li> <li>State 1 means we are inside a normal html tag (the tag buffer contains <code>&lt;</code>)</li> <li>State 2 means we are inside a php tag</li> <li>State 3: we came from the output state and encountered the <code>&lt;</code> and <code>!</code> characters (the tag buffer contains <code>&lt;!</code>)</li> <li>State 4: inside HTML comment</li> </ul> <p>We need just to be careful that no tag can be inserted. That is, <code>&lt;</code> followed by a non-whitespace character. <a href="http://lxr.php.net/opengrok/xref/PHP_5_3/ext/standard/string.c#4326" rel="noreferrer">Line 4326</a> checks an case with the <code>&lt;</code> character which is described below:</p> <ul> <li>If inside quotes (e.g. <code>&lt;a href="inside quotes"&gt;</code>), the <code>&lt;</code> character is ignored (removed from the output).</li> <li>If the next character is a whitespace character, <strong><code>&lt;</code> is added to the output buffer</strong>.</li> <li>if outside a HTML tag, the state becomes <code>1</code> ("inside HTML tag") and the last character <code>lc</code> is set to <code>&lt;</code></li> <li>Otherwise, if inside the a HTML tag, the counter named <code>depth</code> is incremented and the character ignored.</li> </ul> <p>If <code>&gt;</code> is met while the tag is open (<code>state == 1</code>), <code>in_q</code> becomes <code>0</code> ("not in a quote") and <code>state</code> becomes <code>0</code> ("not in a tag"). <strong>The tag buffer is discarded.</strong></p> <p>Attribute checks (for characters like <code>'</code> and <code>"</code>) are done on the tag buffer which is discarded. So the conclusion is: </p> <blockquote> <p>strip_tags without a tag whitelist is safe for inclusion outside tags, no tag will be allowed.</p> </blockquote> <p>By "outside tags", I mean not in tags as in <code>&lt;a href="in tag"&gt;outside tag&lt;/a&gt;</code>. Text may contain <code>&lt;</code> and <code>&gt;</code> though, as in <code>&gt;&lt; a&gt;&gt;</code>. The result is not valid HTML though, <code>&lt;</code>, <code>&gt;</code> and <code>&amp;</code> need still to be escaped, especially the <code>&amp;</code>. That can be done with <a href="http://php.net/htmlspecialchars" rel="noreferrer"><code>htmlspecialchars()</code></a>.</p> <p>The description for <code>strip_tags</code> without an whitelist argument would be:</p> <blockquote> <p>Makes sure that no HTML tag exist in the returned string.</p> </blockquote>
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