Only allowing <a></a>? You may have to use preg_match to find those that aren't <a.?>.?<\/a> tags 😉
i.e.:
$string = '<a href="www.mysite.com">A Link</a><b>Some bold text</b><p>A Paragraph</p><a>A blank Link</a>';
$pattern = '#\<a.*?\>(.*?)\<\/a\>#si';
$replace = '$1';
echo $string.'<br />
';
echo preg_replace($pattern, $replace, $string);
That produces:
<a href="www.mysite.com">A Link</a><b>Some bold text</b><p>A Paragraph</p><a>A blank Link</a><br />
A Link<b>Some bold text</b><p>A Paragraph</p>A blank Link