Actually the ability to mix PHP and "HTML" is the only reason I still use PHP occasionally. Sometimes, I have a plain-text file, and would like to use variables or simple loops in it. The best way to do this is PHP!

For example, say I want to plot a couple of data columns with gnuplot (an open source plotting program) into several images. I can write a PHP script "myplot.plt.php" that generates a gnuplot script (untested):

    set terminal png enhanced
    <?php
    for ($i = 2; $i <= 4; $i++) {
        $outputfile = "plot$i.png"
        echo <<EOT
        set output "$outputfile"
        plot "data.txt" u 1:$i
    EOT;
    }
    ?>

and then invoke it from gnuplot with

    load "< php myplot.plt.php"

This is a stupid example, but it gets really powerful when you dynamically calculate labels, element positions, etc.. PHP may be a crappy language for web development, but it is a really nice macro language, and has really powerful string processing features, making it ideal for this kind of hack.