PHP Fatal error: Uncaught Error: Class 'Monolog\Handler\StreamHandler' not found

composer dump-autoload will make the existing classes available. But does the Class 'Monolog\Handler\StreamHandler' exist? You could check that in the vendor directory.

Now what is the effect of the minimum-stability property? It affects dependency resolution, that means how composer determines which package versions to install. I guess in this case it's about a difference between versions of the Monolog package. The class is expected in it but the installed version somehow does not include it.

That's my guess where I'd look for the cause of the error.

If you want to try to set minimum-stability and your composer.json does not have it, you could add it.

If it doesn't help, could you please look into composer.lock and see which version of Monolog is installed?

Thanks @michapietsch for your valuable response.

But it doesn't work for me.

As i check composer.lock file

monolog version is "monolog/monolog": "~1.12"

and class 'Monolog\Handler\StreamHandler' doesn't exists.

Laravel which version you used?

You can solve this issue by changing "minimum-stability" from "dev" to "stable" in composer.json. Then running composer update.

@ishatanjeeb, Laravel Framework 5.5.44

I don't have monolog files in my project.

@gurvindersingh please share your composer.json file

@Ishatanjeeb please check my coposer.json file This line "minimum-stability": "stable", i added manually after facing this issue.

{
    "name": "laravel/laravel",
    "description": "The Laravel Framework.",
    "keywords": ["framework", "laravel"],
    "license": "MIT",
    "type": "project",
    "minimum-stability": "stable", 
    "require": {
        "php": ">=7.0.0",
        "fideloper/proxy": "~3.3",
        "laravel/framework": "5.5.*",
        "laravel/tinker": "~1.0"
    },
    "require-dev": {
        "filp/whoops": "~2.0",
        "fzaninotto/faker": "~1.4",
        "laravel/homestead": "^7.0",
        "mockery/mockery": "~1.0",
        "phpunit/phpunit": "~6.0"
    },
    "autoload": {
        "classmap": [
            "database/seeds",
            "database/factories"
        ],
        "psr-4": {
            "App\": "app/"
        }
    },
    "autoload-dev": {
        "psr-4": {
            "Tests\": "tests/"
        }
    },
    "extra": {
        "laravel": {
            "dont-discover": [
            ]
        }
    },
    "scripts": {
        "post-root-package-install": [
            "@php -r \"file_exists('.env') || copy('.env.example', '.env');\""
        ],
        "post-create-project-cmd": [
            "@php artisan key:generate"
        ],
        "post-autoload-dump": [
            "Illuminate\Foundation\ComposerScripts::postAutoloadDump",
            "@php artisan package:discover"
        ]
    },
    "config": {
        "preferred-install": "dist",
        "sort-packages": true,
        "optimize-autoloader": true
    }
}

So according to your composer.lock you have version 1.12 installed. I just checked the Github repo for Monolog, chose the 1.12 tag and there the Class exists: https://github.com/Seldaek/monolog/blob/1.12.0/src/Monolog/Handler/StreamHandler.php

Can you confirm, that in the vendor directory, amongst the monolog files you can see this class?

To doubt the mere existence of this class file might seem weird, but who knows what weird things could happen? :)