<?php
 
$string = 'Lorem ipsum <a href="#">dolor</a> sit amet
<div>Consectetur adipisicing elit, sed do</div> 
eiusmod tempor incididunt ut labore et dolore 
magna aliqua.';
 
$test = 'Lorem ipsum dolor sit amet, consectetur adipiscing elit. 
Aliquam sagittis augue in quam mollis egestas. 
Quisque elit erat, luctus sit amet pellentesque non, tempus a ipsum. 
<a href="#">Aenean congue dui at nisl pulvinar porta.</a> 
<strong>Proin id porttitor nisi. Curabitur sed dolor metus, a ornare eros.</strong>
<ul>
    <li>Vestibulum tristique tellus eu nisl feugiat ut convallis massa dictum.</li>
    <li>Vestibulum nisi est, ullamcorper eget vulputate nec, dictum vel felis.</li>
</ul>
In iaculis felis vitae justo consectetur ornare.
Ut porttitor sem in dui elementum tincidunt.
Pellentesque vel malesuada velit.
Nam at mi in ligula congue suscipit.
Aliquam a sollicitudin tellus.';
 
function add_p($string, $delimiter = "\n") {
    // kill it before it starts if it's not a string
    if(!is_string($string)) {
        die('Input must be string');
    }
 
    $string = explode($delimiter, $string);
    $result = '';
    foreach($string as $value) {
        if(preg_match("/^<\\/?div|^<\\/?ul|^<\\/?li|<\\/?p/", $value) >= 1) {
            $result .= $value . "\n";
        } else {
            $result .= '<p>' . trim($value) . '</p>'."\n";
        }
    }
 
    return $result;
 
}
 
echo add_p($string);
 
echo "\n\n";
 
echo add_p($test);

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