Codeigniter SELECT query WHERE a combination of user-submitted filters

try this model

<?php if (!defined('BASEPATH')) exit('No direct script access allowed');

class Model_example extends CI_Model
{

    function __construct()
    {
        parent::__construct();
    }

    public function did_filter()
    {
        $query = $this->db->get_where('table_example',array('phone' => $this->input->post('phone'),'types' => $this->input->post('types')));

        return $query->result_array();   
    }
}

1. A model method should not be designed to directly access user input (superglobals). With very few exceptions, all data required by the model method should be passed in via method parameters or accessed as a class properties (depending on the implemented design pattern). Directly accessing superglobals/inputs, reduces the utility of a method and makes unit testing more difficult.

2. Generally, model methods should not perform data preparations (the same rule applies to views). The lone responsibility of the model layer is to interact with data storage. The model should not perform business logic operations.

3. Try to keep business logic processing and data preparations inside the controller method to a minimum -- "skinny" (brief/short/concise) controller methods should be the goal. The best controller methods look like a list of commands which are performed by other layers. Very seldom should controller methods "do any processing" themselves. Libraries can be a great place to write business logic operations. It is entirely reasonable for controller and library methods to call other layers such as models, libraries, and helpers. Helpers can be utilized for general data processing which is absent of any business logic.

4. Design your methods with the intention of being called from multiple call sites in your application. Unless sacrificing utility for performance for specific cases, design your model methods for flexibility and maintainability.

Suggested model method:

public function get(array $brands = [], array $types = []): array
{
    if ($brands) {
        // one of the selected brands in the phone column
        $this->db->where_in('phone', $brands);
    }
    if ($types) {
        // 1 in at least one of the selected "g" columns
        $this->db->where_in(1, $types, false);
    }
    return $this->db
        ->get('table_example')
        ->result_array();
}

Notes:

• select('*') never needs to be written in CodeIgniter because that is the default SELECT clause.
• Since get() (as well as get_where()) can receive the tablename and is going to be called to execute the built query, omit the from() method from the call chain.
• Although this script does not require any explicit value or identier quoting, see How to select rows where a single value is found in one of multiple columns using CodeIgniter's query-builder methods for the atypical implementation that uses explicit quoting with where_in() when the first parameter is the column value and the second parameter is an array of column names.
• Do not conditionally return false from a method that ordinarily returns an array. This (too common) practice would require call sites to mitigate the possibility of processing a non-array data type before traversing the payload with an iterator. result_array() returns an array of zero or more arrays; if you want to check for an empty array, just make a falsey check on the array.

Your controller method can look like this:

public function search()
{ 
    $this->load->library('form_validation');
    $this->form_validation->set_rules('phone', 'Phone', 'required');
    $this->form_validation->set_rules('types[]', 'Network Type', 'required|in_list[2g,3g,4g]');
    
    if ($this->form_validation->run()) {
        $this->load->model('model_example', 'ModelExample');
        $data['result'] = $this->ModelExample->get(
            (array)$this->input->post('phone'),
            $this->input->post('types')
        );
        if ($data['result']) {
            $this->load->view('view_search_results', $data); 
        } else {
            $this->load->view("view_no_search_results");
        }
    }
}

Notes:

• |in_list[2g,3g,4g] was added to set_rules() to validate the checkbox values against a whitelist of values correlating to database column names.
• The model is only loaded if the form submission passed validation. Resources should only be instantiated if/when they are actually needed.
• You might eventually like to make both phone and types form fields into multiselects or checkboxes. When that future comes, your model method will be ready.
• I wouldn't create an additional view file just for "no results", I'd design my results view to accommodate zero or more results.