Laravel use of concat with pluck method

Try changing the eloquent query to:

$ProjectManagers = Employees::select(
            DB::raw("CONCAT(first_name,' ',last_name) AS name"),'id')
            ->where('designation', 1)
            ->pluck('name', 'id');

if your column is nullable then you should try this

convert the NULL values with empty string by COALESCE

$ProjectManagers = Employees::select(
            DB::raw("CONCAT(COALESCE(`first_name`,''),' ',COALESCE(`last_name`,'')) AS name"),'id')
            ->where('designation', 1)
            ->pluck('name', 'id')
            ->toArray();

The most elegant solution is to create an accessor.

Open your Employees class (model) and add an accessor function:

public function getFullNameAttribute()
{
    return $this->first_name . ' ' . $this->last_name;
}

After that, just simply use:

$ProjectManagers = Employees::where('designation', 1)->get()->pluck('full_name', 'id');