<p> и <<p>qwe asd zxc.cvb dddfg. dfgdfg dfgdfg.ddg ertert. dfgdfg<';<p>qwe asd zxc. cvb dddfg. dfgdfg dfgdfg. ddg ertert. dfgdfg<';<?php
$t = '<p>qwe asd zxc.cvb dddfg. dfgdfg dfgdfg.ddg ertert. dfgdfg</p>';
$result = preg_replace('#(<p>)(.*?)(<)#', '$1$3', $t);
echo $result;