Q:

What todo when composer update and composer install give error ?

I run composer install I got this error, telling me to use composer update 
Loading composer repositories with package information
Installing dependencies (including require-dev) from lock file
Warning: The lock file is not up to date with the latest changes in composer.json. You may be getting outdated dependencies. Run update to update them.
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Installation request for doctrine/inflector v1.3.0 -> satisfiable by doctrine/inflector[v1.3.0].
    - doctrine/inflector v1.3.0 requires php ^7.1 -> your PHP version (7.0.25) does not satisfy that requirement.
  Problem 2
    - Installation request for symfony/css-selector v4.0.3 -> satisfiable by symfony/css-selector[v4.0.3].
    - symfony/css-selector v4.0.3 requires php ^7.1.3 -> your PHP version (7.0.25) does not satisfy that requirement.
  Problem 3
    - Installation request for symfony/event-dispatcher v4.0.3 -> satisfiable by symfony/event-dispatcher[v4.0.3].
    - symfony/event-dispatcher v4.0.3 requires php ^7.1.3 -> your PHP version (7.0.25) does not satisfy that requirement.
  Problem 4
    - Installation request for doctrine/instantiator 1.1.0 -> satisfiable by doctrine/instantiator[1.1.0].
    - doctrine/instantiator 1.1.0 requires php ^7.1 -> your PHP version (7.0.25) does not satisfy that requirement.
  Problem 5
    - doctrine/inflector v1.3.0 requires php ^7.1 -> your PHP version (7.0.25) does not satisfy that requirement.
    - laravel/framework v5.5.32 requires doctrine/inflector ~1.1 -> satisfiable by doctrine/inflector[v1.3.0].
    - Installation request for laravel/framework v5.5.32 -> satisfiable by laravel/framework[v5.5.32].

I run composer update, I got this 
Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - doctrine/dbal v2.6.3 requires php ^7.1 -> your PHP version (7.0.25) does not satisfy that requirement.
    - doctrine/dbal v2.6.2 requires php ^7.1 -> your PHP version (7.0.25) does not satisfy that requirement.
    - doctrine/dbal v2.6.1 requires php ^7.1 -> your PHP version (7.0.25) does not satisfy that requirement.
    - doctrine/dbal v2.6.0 requires php ^7.1 -> your PHP version (7.0.25) does not satisfy that requirement.
    - Installation request for doctrine/dbal ^2.6 -> satisfiable by doctrine/dbal[v2.6.0, v2.6.1, v2.6.2, v2.6.3].

Now what should I do ? 
This is what I have in composer.json
cat composer.json 
{
    "name": "laravel/laravel",
    "description": "The Laravel Framework.",
    "keywords": ["framework", "laravel"],
    "license": "MIT",
    "type": "project",
    "require": {
        "php": ">=7.0.0",
        "anouar/paypalpayment": ">=3.0",
        "doctrine/dbal": "^2.6",
        "fideloper/proxy": "~3.3",
        "guzzlehttp/guzzle": "^6.3",
        "intervention/image": "^2.4",
        "laravel/framework": "5.5.*",
        "laravel/tinker": "~1.0",
        "spatie/laravel-newsletter": "^4.1",
        "srmklive/paypal": "~1.0"
    },
    "require-dev": {
        "filp/whoops": "~2.0",
        "fzaninotto/faker": "~1.4",
        "mockery/mockery": "~1.0",
        "phpunit/phpunit": "~6.0"
    },
    "autoload": {
        "classmap": [
            "database/seeds",
            "database/factories"
        ],
        "psr-4": {
            "App\\": "app/"
        },
        "files": ["app/Http/Helper.php"]
    },
    "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
    }
} 

A:

I faced the same issue few days ago I fixed it by faking the php version in my composer.json file [Not recommended].
"config": {
        "preferred-install": "dist",
        "sort-packages": true,
        "optimize-autoloader": true,
        "platform" : {
            "php": "7.1"
        }
    }

For more details read this
Or you can execute composer update/install commands with --ignore-platform-reqs flag as shown below

composer update --ignore-platform-reqs

