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Problem

The value of the checkbox not sent to the POST if it is not selected.

Solution

Make sure the value is set in the $ _POST array before using it.

    $nro_funcoes = 10;

    for ($id = 1; $id <= $nro_funcoes; $id++) {
        $nro_funcao[$id] = $id;
    }

    foreach ($nro_funcao as $func) {

        //verificar se está definido

        if (isset($_POST[$func])) {
            $funcao[$func] = $_POST[$func];
        }
    }

    $dados = implode(', ', array_filter($funcao));