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use Benchmark ':all';

...
    e_double => sub { eval q{"foo"} },
    e_single => sub { eval q{'foo'} },
});

$ perl -MO=Concise -e'$x=q{foo}' > single.txt
-e syntax OK
...
$ diff -s single.txt double.txt
Files single.txt and double.txt are identical
$ rm single.txt double.txt

<?php

...
    "single quotes" => create_function('', 'return \'foo\';'),
    "double quotes" => create_function('', 'return "foo";'),
));