Using PHP replace regex with regex

This should nudge you in the right direction:

echo preg_replace_callback('/#(\w+)/', function($match) {
    return sprintf('<a href="https://www.google.com?q=%s">%s</a>', 
        urlencode($match[1]), 
        htmlspecialchars($match[0])
    );
}, htmlspecialchars($text));

See also: preg_replace_callback()

$input_lines="any word here related to #English must #be replaced.";
preg_replace("/(#\w+)/", "<a href='bla bla'>$1</a>", $input_lines);

DEMO

OUTPUT:

any word here related to <a href='bla bla'>#English</a> must <a href='bla bla'>#be</a> replaced.