Using theme options to change a border-radius value

Use standard echo or printf with additional parameters. e.g.

if(isset($option['borderradius']) && (!empty($option['borderradius']))) {   
    $borderradius=".post { ";
    $borderradius .= 'border-radius: ' . $option['borderradius'] . 'px; ';
    $borderradius .= '-webkit-border-radius: ' . $option['borderradius'] . 'px; ';
    $borderradius .= '-moz-border-radius: ' . $option['borderradius'] . 'px; ';
    $borderradius .= '}';

    echo $borderradius;
}

if(isset($option['borderradius']) && (!empty($option['borderradius']))) { 
    printf('.post { border-radius: %spx; -webkit-border-radius: %spx; -moz-border-radius: %spx; }', $option['borderradius'], $option['borderradius'], $option['borderradius']);
}