Insert after each opening tag
<p>How do I use <code>preg_replace</code> to insert a given string after each opening tag in a string containing HTML? For example, if I have the string:</p>

<pre><code>$str = 'sds&lt;some_tag&gt;ttt&lt;/some_tag&gt;dfg&lt;some_tag&gt;vvv&lt;/some_tag&gt;sdf';
</code></pre>

<p>and I want to insert <code>&lt;b&gt;</code>, the result should be:</p>

<pre><code>$str = 'sds&lt;some_tag&gt;&lt;b&gt;ttt&lt;/some_tag&gt;dfg&lt;some_tag&gt;&lt;b&gt;vvv&lt;/some_tag&gt;sdf';
</code></pre>
