An embedded Perl interpreter for PHP does exist but it's a pretty bizarro thing to do.

Update: the following works here:

<?php

$perl = new Perl();
$perl->eval(<<<'PERL'
sub add_two {
    my $n = shift;
    return $n + 2;
}
PERL
);

echo "add_two(3) is ... ", $perl->add_two(3), "\n";
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Interestingly when running print "$^X $]" in the embedded Perl, I get: /usr/bin/php 5.010001 (that version number happens to correspond to my operating system's pre-packaged version of Perl).