Laravel: file_put_contents() failed to open stream: Permission denied for Session folder

Suggestion from vsmoraes worked for me:

Laravel >= 5.4

php artisan cache:clear 
chmod -R 775 storage/
composer dump-autoload

Laravel < 5.4

php artisan cache:clear 
chmod -R 775 app/storage 
composer dump-autoload

NOTE: DO NOT DO THIS ON ANY REMOTE SERVER (DEV OR PRODUCTION)

When I asked this question, this was a problem on my localhost, running in a Virtual Machine. So I thought setting up a 777 was safe enough, however, folks are right when they say you should look for a different solution. Try 775 first

Doing some deeper looking into ControlBuilder, I noticed that the compiled factory for each control instance will attach a DataBinding event handler when there are data binding expressions present. I've found that checking for this seems to be a very reliable method for determining if data binding needs to occur. Here is the basis of my solution to the problem:

using System;
using System.Reflection;
using System.Web.UI;

public class AutoDataBindControl : Control
{
    private static readonly object EventDataBinding;
    private bool needsDataBinding = false;

    static AutoDataBindControl()
    {
        try
        {
            FieldInfo field = typeof(Control).GetField(
                "EventDataBinding",
                BindingFlags.NonPublic|BindingFlags.Static);

            if (field != null)
            {
                AutoDataBindControl.EventDataBinding = field.GetValue(null);
            }
        }
        catch { }

        if (AutoDataBindControl.EventDataBinding == null)
        {
            // effectively disables the auto-binding feature
            AutoDataBindControl.EventDataBinding = new object();
        }
    }

    protected override void DataBind(bool raiseOnDataBinding)
    {
        base.DataBind(raiseOnDataBinding);

        // flag that databinding has taken place
        this.needsDataBinding = false;
    }

    protected override void OnInit(EventArgs e)
    {
        base.OnInit(e);

        // check for the presence of DataBinding event handler
        if (this.HasEvents())
        {
            EventHandler handler = this.Events[AutoDataBindControl.EventDataBinding] as EventHandler;
            if (handler != null)
            {
                // flag that databinding is needed
                this.needsDataBinding = true;

                this.Page.PreRenderComplete += new EventHandler(this.OnPreRenderComplete);
            }
        }
    }

    void OnPreRenderComplete(object sender, EventArgs e)
    {
        // DataBind only if needed
        if (this.needsDataBinding)
        {
            this.DataBind();
        }
    }
}

This solution disables itself if no DataBinding event handler is attached or if the control is manually data bound (directly or via a parent).

Note that most of this code is just jumping through hoops to be able to test for the existence of the event. The only reflection needed is a one-time lookup to get the object used as the key for EventDataBinding.