PHP: how to reuse code (oop)?

I have studied in php oop and stocked in the concept of reusable code.

I have seen an example like

interface iTemplate
{
    public function setVariable($name, $var);
    public function getHtml($template);
}
And implement it:

// Implement the interface
class Template implements iTemplate
{
    private $vars = array();

    public function setVariable($name, $var)
    {
        $this->vars[$name] = $var;
    }

    public function getHtml($template)
    {
        foreach($this->vars as $name => $value) {
            $template = str_replace('{' . $name . '}', $value, $template);
        }

        return $template;
    }
} 

I can understand the code but not sure why it is reusable. Every time I want to add a new function in iTemplate interface, my Template class needs to be changed too. I don't understand the concept of "reuse". I appreciate any help. Thanks.

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