<?php
$dec = 0.625;

$hours = $dec * 24;

function convertTime($dec)
{
    // start by converting to seconds
    $seconds = ($dec * 3600);
    // we're given hours, so let's get those the easy way
    $hours = floor($dec);
    // since we've "calculated" hours, let's remove them from the seconds variable
    $seconds -= $hours * 3600;
    // calculate minutes left
    $minutes = floor($seconds / 60);
    // remove those from seconds as well
    $seconds -= $minutes * 60;
    // return the time formatted HH:MM:SS
    return lz($hours).":".lz($minutes).":".lz($seconds);
}

// lz = leading zero
function lz($num)
{
    return (strlen($num) < 2) ? "0{$num}" : $num;
}

echo convertTime($hours);