PHP uses double-precision floating point numbers. Neither of the results of the two logarithms can be represented exactly, so the result of dividing them is not exact. The result you get is close to, but slightly less than 3. This gets rounded to 3 when being formatted by echo. floor, however returns 2.

You can avoid the inexact division by taking advantage of the fact that log(x, b) / log(y, b) is equivalent to log(x, y) (for any base b). This gives you the the expression log(238328, 62) instead, which has a floating point result of exactly 3 (the correct result since 238328 is pow(62, 3)).

The functions give different results with negative fractions.

echo floor(-0.1); // -1
echo intval(-0.1); // 0

floor() will simply drop decimal value and return only integer.

So floor(1.2) => 1 and floor(1.9) => 1.

Meanwhile round() will round number that has decimal value lower than 0.5 to lower int, and when more than 0.5 to higher int:

So round(1.2) => 1 but round(1.9) => 2

Also round() has more options, like precision and rounding options.

Example:

$nums = [-1.5, -1, -.8, -.4, 0, .4, .8, 1, 1.5];

echo "    \tround\tfloor\tceil" . PHP_EOL;
foreach ($nums as $a) {
    echo $a . ": \t" . round($a) . "\t" . floor($a) . "\t" . ceil($a) . PHP_EOL;
}

/*
        round   floor   ceil
-1.5:   -2      -2      -1
  -1:   -1      -1      -1
-0.8:   -1      -1      -0
-0.4:   -0      -1      -0
   0:    0       0       0
 0.4:    0       0       1
 0.8:    1       0       1
   1:    1       1       1
 1.5:    2       1       2

*/