Using passthru("whatever") is almost 100 times slower than doing the equivalent $out = `whatever`;echo $out

I tested this using the following program (I use dd here to generate the data, but the same results appear with any command, cat /etc/passwd etc.)

<?php
        echo "<!--";
        $start = microtime();
        passthru("dd if=/dev/zero count=100");
        $end = microtime();
        echo "-->";

        $mt = explode(' ', $start);
        $mts = $mt[1] + $mt[0];
        $mt = explode(' ', $end);
        $mte = $mt[1] + $mt[0];
        printf("%.3f seconds elapsed (passthru)<p>", $mte - $mts);

        echo "<!--";
        $start = microtime();
        $out = `dd if=/dev/zero count=100`;
        echo $out;
        $end = microtime();
        echo "-->";

        $mt = explode(' ', $start);
        $mts = $mt[1] + $mt[0];
        $mt = explode(' ', $end);
        $mte = $mt[1] + $mt[0];
        printf("%.3f seconds elapsed (backquotes)<p>", $mte - $mts);
?>

On my machine (Pentium 400) this script outputs:

16.231 seconds elapsed (passthru)
0.266 seconds elapsed (backquotes)

Quite a difference, I think.

Compiled with:

./configure \
        --bindir=/usr/local/apache/bin \
        --libdir=/usr/local/apache/php/lib \
        --with-apxs=/usr/local/apache/bin/apxs \
        --with-config-file-path=/usr/local/apache/conf \
        --with-exec-dir=/usr/local/apache/php/bin \
        --with-pear=/usr/local/apache/php/lib \
        --enable-safe-mode \
        --with-openssl=/usr/local/ssl \
        --with-imap=shared,/usr/local \
        --enable-dbx=shared \
        --enable-dio=shared \
        --enable-ftp \
        --with-sybase-ct=shared,/usr/local \
        --with-mysql=shared,/usr/local/mysql \
        --with-pgsql=shared,/usr/local/pgsql \
        --with-gdbm=shared \
        --with-zlib=/usr/local \
        --with-gd=shared \
        --enable-exif=shared \
        --enable-ctype \
        --with-gettext \
        --with-xml=shared \
        --with-expat-dir=/usr/local \
        --enable-xslt=shared \
        --with-xslt-sablot=/usr/local \
        --enable-sockets=shared \
        --enable-wddx=shared \
        --enable-shmop=shared \
        --enable-sysvmsg=shared \
        --enable-sysvsem=shared \
        --enable-sysvshm=shared