laravel/app/Http/Middleware/VerifyCsrfToken.php

<?php namespace App\Http\Middleware;

use Closure;
use Illuminate\Contracts\Routing\Middleware;
use Illuminate\Session\TokenMismatchException;

class VerifyCsrfToken implements Middleware {

	/**
	 * Handle an incoming request.
	 *
	 * @param  \Illuminate\Http\Request  $request
	 * @param  \Closure  $next
	 * @return mixed
	 *
	 * @throws TokenMismatchException
	 */
	public function handle($request, Closure $next)
	{
		if ($this->isReading($request) || $this->tokensMatch($request))
		{
			return $next($request);
		}

		throw new TokenMismatchException;
	}

	/**
	 * Determine if the session and input CSRF tokens match.
	 *
	 * @param  \Illuminate\Http\Request  $request
	 * @return bool
	 */
	protected function tokensMatch($request)
	{
		return $request->session()->token() == $request->input('_token');
	}

	/**
	 * Determine if the HTTP request uses a ‘read’ verb.
	 *
	 * @param  \Illuminate\Http\Request  $request
	 * @return bool
	 */
	protected function isReading($request)
	{
		return in_array($request->method(), ['HEAD', 'GET', 'OPTIONS']);
	}

}