why composer ignore my php version

Question

I have a silex project where my server have PHP 5.3.3, update PHP is not on the table.

{
    "config": {
        "platform": {
            "php": "5.3.3"
        }
    },
    "require": {
        "silex/silex": "1.2.*",
        "twig/twig": ">=1.8.0,<2.0-dev",
        "symfony/twig-bridge": "~2.3,<2.7",
        "twig/extensions": "1.2.*@dev",
        "monolog/monolog": "~1.4,>=1.4.1",
        "vrana/notorm": "dev-master",
        "mobiledetect/mobiledetectlib": "dev-master",
        "symfony/security": "2.6.*"
    }
}

My problem is that symfony/security install version 2.6.13 and in dependencies there is this line symfony/http-kernel: ~2.4

that install symfony/http-kernel 2.6.16 and symfony/http-kernel 2.6.16 have like requirement symfony/debug: ~2.6,>=2.6.2 but instead install symfony/debug: 2.6.16 composer install symfony/debug: 2.8.15 and that version require PHP 5.3.9

here is the error

Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Installation request for symfony/debug v2.8.15 -> satisfiable by symfony/debug[v2.8.15].
    - symfony/debug v2.8.15 requires php >=5.3.9 -> your PHP version (5.3.3) does not satisfy that requirement.
  Problem 2
    - symfony/debug v2.8.15 requires php >=5.3.9 -> your PHP version (5.3.3) does not satisfy that requirement.
    - symfony/http-kernel v2.6.13 requires symfony/debug ~2.6,>=2.6.2 -> satisfiable by symfony/debug[v2.8.15].
    - Installation request for symfony/http-kernel v2.6.13 -> satisfiable by symfony/http-kernel[v2.6.13].

How can I fix that?

Guys, the problem is this line

- symfony/http-kernel v2.6.13 requires symfony/debug ~2.6,>=2.6.2 -> satisfiable by symfony/debug[v2.8.15].

look the debug version that want and the version that get

Answer 1

I solve it deleting the file called composer.lock and executing

composer clearcache
composer update