How come a simple PHP array lookup is so slow? Am I doing something wrong here?

How come a simple PHP array lookup is so slow? Am I doing something wrong
here?

I have the following function:
function percentToColor($percent){
$minBrightness = 160;
$maxBrightness = 255;
// Remainins?
$brightness =
((($minBrightness-$maxBrightness)/(100-0))*$percent+$maxBrightness);
$first = (1-($percent/100))*$brightness;
$second = ($percent/100)*$brightness;
// Find the influence of the middle color (yellow if 1st and 2nd are
red and green)
$diff = abs($first-$second);
$influence = ($brightness-$diff)/2;
$first = intval($first + $influence);
$second = intval($second + $influence);
// Convert to HEX, format and return
$firstHex = str_pad(dechex($first),2,0,STR_PAD_LEFT);
$secondHex = str_pad(dechex($second),2,0,STR_PAD_LEFT);
return $firstHex . $secondHex . '00';
}
This function accepts integers ranging from 0 to 100 and returns the color
information for that number. Imagine a progress bar with red at 0 and
green at 100. This is what the function does.
So I get the idea: if this function always returns the same color for each
input (i.e. the colors aren't time/user/session dependent), the better
idea would be to create a PHP array with the results, right?
So I rewrite the function:
function percentToColorNew($percent){
$results = array(
0 => 'ff0000',
1 => 'fe0500',
2 => 'fd0a00',
3 => 'fc0f00',
// ... continues with 4, 5, up until 100 ...
99 => '03a000',
100 => '00a000'
);
return $results[$percent];
}
And I test it. And the unexpected happens! The new function, which only
returns the result from the array of results, takes double the time as the
original function which has to compute the result every time it's called.
Why is this? Are PHP arrays this slow? Is there a faster way of storing
the function results to speed up the function? A switch, maybe?
If/elseif/else conditions? A different way of working with arrays?