The phpmd command needs at least one ruleset. If there is no ruleset, phpmd doesn't run. The result was below:

    $ phpmd test/resources/checkers/php-phpmd-warning.php text
    Mandatory arguments:
    1) A php source code filename or directory. Can be a comma-separated string
    2) A report format
    3) A ruleset filename or a comma-separated string of rulesetfilenames

    Available formats: xml, text, html.
    Available rulesets: cleancode, codesize, controversial, design, naming, unusedcode.

    Optional arguments that may be put after the mandatory arguments:
    --minimumpriority: rule priority threshold; rules with lower priority than this will not be used
    --reportfile: send report output to a file; default to STDOUT
    --suffixes: comma-separated string of valid source code filename extensions
    --exclude: comma-separated string of patterns that are used to ignore directories
    --strict: also report those nodes with a @SuppressWarnings annotation