Detectar keywords de pagina web

[prcarreno]

// Get a file into an array. In this example we'll go through HTTP to get
// the HTML source of a URL.
$lines = file('http://www.paginaweb.com/');

// Loop through our array, show HTML source as HTML source; and line numbers too.
foreach ($lines as $line_num => $line) {
echo "Line #<b>{$line_num}</b> : " . htmlspecialchars($line) . "<br />\n";
}

puedes hacer un substr_count($line,"keyword"); para saber si la linea contiene lo que buscas.
Es lo que se me ocurre xD