round(3.78 * 5) / 5 = 3.8

function roundToNearestFraction( $number, $fractionAsDecimal )
{
     $factor = 1 / $fractionAsDecimal;
     return round( $number * $factor ) / $factor;
}

// Round to nearest fifth
echo roundToNearestFraction( 3.78, 1/5 );

// Round to nearest third
echo roundToNearestFraction( 3.78, 1/3 );

function round2($original) {
    $times5 = $original * 5;
    return round($times5) / 5;
}