Untitled

echo preg_replace('#<hr *id="system-readmore" */>.*#is','',$this->row->product_description);

// this is your $this->row->product_description value for demonstration purposes:
$html = <<<HTML
Some text yatta yatta
<hr id="system-readmore" />
Some more text blah blah
HTML;

list($introtext, $description) = explode('<hr id="system-readmore" />', $html, 2);
var_dump($introtext);
var_dump($description);

echo "n---n";

var_dump(trim($introtext));  // this is tidier, with leading/trailing whitespace characters removed
var_dump(trim($description));  // this is tidier, with leading/trailing whitespace characters removed

echo "n---n";

list($introtext, $description) = preg_split('~s*<hr id="system-readmore" />s*~', $html, 2);
var_dump($introtext);  // no trim() call, no leading/trailing whitespace characters displayed
var_dump($description);  // no trim() call, no leading/trailing whitespace characters displayed

string(22) "Some text yatta yatta
"
string(25) "
Some more text blah blah"

---
string(21) "Some text yatta yatta"
string(24) "Some more text blah blah"

---
string(21) "Some text yatta yatta"
string(24) "Some more text blah blah"