<p>If you have the following:</p>

<pre><code>$var = 3; // we'll say it's set to 3 for this example
if ($var == 4) {
    // do something
} else if ($var == 5) {
    // do something
} else if ($var == 2) {
    // do something
} else if ($var == 3) {
    // do something
} else {
    // do something
}
</code></pre>

<p>If say 80% of the time <code>$var</code> is 3, do you worry about the fact that it's going through 4 if cases before finding the true case?</p>

<p>I'm thinking on a small site it's not a big deal, but what about when that if statement is going to run 1000s of times a second?</p>

<p>I'm working in PHP, but I'm thinking the language doesn't matter.</p>

A classic case of this occurring (with literally 5 options as in your post) was in ffmpeg, in the decode_cabac_residual function.  This was rather important, as profiling (very important--don't optimize before profiling!) showed it counted for upwards of 10-15% of the time spent in H.264 video decoding.  The if statement controlled a set of statements which was calculated differently for the various types of residuals to be decoded--and, unfortunately, too much speed was lost due to code size if the function was duplicated 5 times for each of the 5 types of residual.  So instead, an if chain had to be used.
Profiling was done on many common test streams to order them in terms of likelihood; the top was the most common, the bottom the least.  This gave a small speed gain.
Now, in PHP, I suspect that there's a lot less of the low-level style speed gain that you'd get in C, as in the above example.