preg replace

preg_replace

Replaces the matches in the subject with the specified replacement.

$text = 'The definition of [recursion] may not be [obvious].';
echo preg_replace('/[([^]]+)]/', '<b>1</b>', $text);
$text = 'The definition of <h1>recursion</h1> may not be <h1>obvious</h1>.';
echo preg_replace('@<h1[^>]*?>.*?</h1>@siu', '', $text);
$str ='asdfd adsfd aaaadasd'; 
echo $str = preg_replace('/(a)(.*)?(d)/U','a($2)d',$str);