将onclick事件echo << EOF转换为简单回声

I have the following echo <<EOF onclick event and want to convert it to simple echo, How can I do it:

    echo <<<EOF
     <button type="button"  
onclick="location.href=((document.getElementById('{$product->get_id()}')!=null) ? document.getElementById('{$product->get_id()}').value : '{$product->add_to_cart_url()}');" >
Add To Cart </button>
        EOF;

I tried concatenating but it didn't work.

How can I echo the onclick event with simple echo instead of echo <<<EOF...EOF;?

Heredoc is useful when you have a string containing both ' and ".

Having said that, here you go, just escape with backslashes:

echo "<button type=\"button\" onclick=\"location.href=((document.getElementById('" . $product->get_id() . "')!=null) ? document.getElementById('" . $product->get_id() . "').value : '" . $product->add_to_cart_url() . "');\" >
Add To Cart </button>";