$str = '<iframe width="560" height="315" src="//www.youtube.com/embed/abcdef" frameborder="0" allowfullscreen></iframe>';

return $str;
// '<iframe width=&#8221;560&#8243; height=&#8221;315&#8243; src=&#8221;//www.youtube.com/embed/abcdef&#8221; frameborder=&#8221;0&#8243; allowfullscreen></iframe>'

$str = htmlentities ($content, ENT_QUOTES);
    // '&lt;iframe width=&#8221;560&#8243; height=&#8221;315&#8243; src=&#8221;//www.youtube.com/embed/abcdef&#8221; frameborder=&#8221;0&#8243; allowfullscreen&gt;&lt;/iframe&gt;'

    $str = html_entity_decode( $str );
    // '<iframe width=&#8221;560&#8243; height=&#8221;315&#8243; src=&#8221;//www.youtube.com/embed/abcdef&#8221; frameborder=&#8221;0&#8243; allowfullscreen></iframe>'