Use PHP Document Object Model:
<?php
$str = '<h1>T1</h1>Lorem ipsum.<h1>T2</h1>The quick red fox...<h1>T3</h1>... jumps over the lazy brown FROG';
$DOM = new DOMDocument;
$DOM->loadHTML($str);
//get all H1
$items = $DOM->getElementsByTagName('h1');
//display all H1 text
for ($i = 0; $i < $items->length; $i++)
echo $items->item($i)->nodeValue . "<br/>";
?>
This outputs as:
T1
T2
T3
[EDIT]: After OP Clarification:
If you want the content like Lorem ipsum. etc, you can directly use this regex:
<?php
$str = '<h1>T1</h1>Lorem ipsum.<h1>T2</h1>The quick red fox...<h1>T3</h1>... jumps over the lazy brown FROG';
echo preg_replace("#<h1.*?>.*?</h1>#", "", $str);
?>
this outputs:
Lorem ipsum.The quick red fox...... jumps over the lazy brown FROG
T1, T2, T3but the text after one heading ends and before next heading begins. e.g.Lorem ipsum.. So, this is different. Please take a look.