even though they said Tested with Laravel 5.5.* it in the readme,composer.json says it requires both
"laravel/framework": "5.4.*",
"illuminate/support": "5.4.*",
so if the package's composer.json says it should be Laravel 5.4.*. You are using laravel 5.5 so it's incompatible.
here is the rpsimao/invoicexpress-api composer.json file of tag 0.5.6. see Laravel version
{
"name": "rpsimao/invoicexpress-api",
"description": "Laravel Package to interact with InvoiceXpress API",
"type": "library",
"keywords": [
"laravel",
"rpsimao",
"invoicexpress-api",
"invoicexpress",
"api"
],
"homepage": "https://github.com/rpsimao/invoicexpress-api",
"license": "MIT",
"authors": [
{
"name": "Ricardo Simão",
"email": "ricardo.simao@upgrade.pt"
}
],
"require": {
"php": "^7.0",
"guzzlehttp/guzzle": "^6.2.1",
"guzzlehttp/psr7": "^1.4.1",
"guzzlehttp/promises": "~1.0",
"laravel/framework": "5.4.*",
"illuminate/support": "5.4.*",
"spatie/array-to-xml": "^2.5"
},
"autoload": {
"psr-4": {
"rpsimao\\InvoiceXpressAPI\\": "src/"
},
"files":[
"src/Helpers/invoicexpress-api.php"
]
},
"require-dev": {
"phpunit/phpunit": "^6.1",
"orchestra/testbench": "^3.4"
},
"autoload-dev": {
"psr-4": {
"rpsimao\\InvoiceXpressAPI\\Test\\": "tests/"
}
},
"extra": {
"laravel": {
"providers": [
"rpsimao\\InvoiceXpressAPI\\InvoiceXpressAPIServiceProvider"
],
"aliases": {
"InvoiceXpressClients": "rpsimao\\InvoiceXpressAPI\\InvoiceXpressAPIFacade"
}
}
}
}
AFTER EDIT
So, currently it's not possible to use with Laravel 5.5 as Laravel 5.5 uses PHP >=7.0.0 too, this package uses Laravel 5.4 which uses PHP >=5.6.4.
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