How to make replacement, only replacing once

[basketmen]

Hi guys, below are custom code to replacing text into link, for example

Google text, will be like this Google
Yahoo text, will be like this Yahoo

the replacement mod below are working well until now

now i just need a little fix again, to make it better again
i need it now only replacing once, so not too many links if the text appear few times

please share your knowledge guys, how to do that

$words = array(
    'Google' => 'http://www.google.com',
    'Yahoo' => 'http://www.yahoo.com'
);

$exp = array();
$rplc = array();
foreach ($words as $word => $url)
{
    $exp[] = '/ ' . preg_quote($word) . ' /i';
    $rplc[] = ' <argh href="' . $url . '">' . $word . '</argh> ';
}

    $this->post['message'] = preg_replace($exp, $rplc, $this->post['message']);
    $this->post['message'] = preg_replace_callback("#(<a [^>]*>)((?:[^<]|<(?!/?a>)|(?R))+)(</a>)#", create_function('$a', 'return $a[1] . preg_replace("#<argh[^>]*>(.*?)</argh>#", "$1", $a[2]) . $a[3];'), $this->post['message']);
    $this->post['message'] = preg_replace(array('#<argh #', '#</argh>#'), array('<a ', '</a>'), $this->post['message']); 

[Force Flow]

Use the limit parameter: PHP: preg_replace - Manual