Array to string conversion in Php Codeigniter

Solution:

You are sending an array/object into your getTasksDetails() function when that function should receive a string.

This line here:

$project_name = $this->project_model->getProjectDetails($project_manager);
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Should be:

$project_name = $this->project_model->getProjectDetails($project_manager[0]->project_manager);
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However I would go even further and change your function that returns the project manager to return just one row instead of result.

From what I see I suspect you're returning $foo->result() on that function, instead change that to $foo->row();

Then instead of using

$project_manager[0]->project_manager 
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you would just use:

$project_manager->project_manager
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This solution is obviously based on the fact that you might want projects details for just one project manager.

If what you want is all the project details for a group of project managers then you might want to tweak your project managers array into being just one array of ids and then change your query to use where in instead of where. So I just assume your function getmanager is probably wrong since you're sending a user_id and getting more than one result.

Based on your comments, saying that what you need is all the tasks associated with all the projects of a given project manager, what you need is the following:

// Controller Function
function index()
{
    $user_id =  $this->session->userdata('manager');
    // return the project manager name
    $project_manager = $this->task_model->getmanager($user_id);
    // Return all the projects for a given project manager
    $project_manager_projects = $this->task_model->getProjectDetails($project_manager); 
    // Get all tasks in a set of projects
    $data['tasks'] = $this->task_model->getTasksDetails($project_manager_projects);
}


// Model functions
function getmanager($user_id)
{
    $first_name =$this->db->select('first_name')
        ->from('user_login')
        ->where(array('id' => $user_id,'delete_flag'=>0))
        ->get()
        ->row();
    return  $first_name->first_name; 
} 

function getProjectDetails($project_manager)
{
    $delete_flag=1;
    $project_name = $this->db->get_where('projects',array(
        'delete_flag != ' => $delete_flag,
        'project_manager' => $project_manager)
    )->result();
    return  $project_name;
}


function getTasksDetails($project_manager_projects)
{
    $projects = array();
    foreach($project_manager_projects as $project) {
        $projects[] = $project->project_name;
    }
    if (empty($projects)) {
        return array();
    }
    return $this->db->select('*,tasks.id as id, tasks.status as status')
        ->join('user_login', 'tasks.assign_to = user_login.id')
        ->where('tasks.delete_flag','0')
        ->where_in('tasks.project_name' ,$projects)
        ->get('tasks')
        ->result(); 
} 
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For your where_in to work, you need to use an array with just the data you are using, not an entire multidimensional array with a lot of fields that aren't used.

Now this code can be improved quite a lot, but to solve your question this should suffice.