I have a Symfony (4.2) project and need to setup/communicate with a web-service.
So after a short search I found that there is a package for Symfony which should makes handling of web-services easy.
It's called "API Platform".

On Symfonycast I found a video of how to install the API-Platform Symfonycast

Sounds easy, so let's do it by easily run the command

composer require api:1.2.0

to install api-platfrom via composer

... and it fails with the message:

require.api is invalid, it should have a vendor name, a forward slash, and a package name. The vendor and package name can be words separated by -, . or _. The complete name should match "^[a-z0-9]([_.-]?[a-z0-9]+)*/[a-z0-9](([_.]?|-{0,2})[a-z0-9]+)*$". 

For me it seems that it did not find the api recipe. So on the github page of the Symfony/flex Bundle I found the List of existing recipes RecipeList and in it there was no "api" recipe only a "api-platform/core" recipe. So I tryed that with:

composer require api-platform/core

... and it fails again this time with another error message:

Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - symfony/flex is locked to version v1.6.2 and an update of this package was not requested.
    - symfony/flex v1.6.2 requires composer-plugin-api ^1.0 -> found composer-plugin-api[2.3.0] but it does not match the constraint.
  Problem 2
    - api-platform/core[v2.6.0, ..., v2.6.6] require symfony/http-foundation ^4.4 || ^5.1 -> found symfony/http-foundation[v4.4.0, ..., v4.4.42, v5.1.0, ..., v5.4.9] but the package is fixed to v4.2.12 (lock file version) by a partial update and that version does not match. Make sure you list it as an argument for the update command.
    - api-platform/core[v2.6.7, ..., v2.6.8] require symfony/http-foundation ^4.4 || ^5.1 || ^6.0 -> found symfony/http-foundation[v4.4.0, ..., v4.4.42, v5.1.0, ..., v5.4.9, v6.0.0, ..., v6.1.1] but the package is fixed to v4.2.12 (lock file version) by a partial update and that version does not match. Make sure you list it as an argument for the update command.  
    - Root composer.json requires api-platform/core ^2.6 -> satisfiable by api-platform/core[v2.6.0, ..., v2.6.8].

Use the option --with-all-dependencies (-W) to allow upgrades, downgrades and removals for packages currently locked to specific versions.
You can also try re-running composer require with an explicit version constraint, e.g. "composer require api-platform/core:*" to figure out if any version is installable, or "composer require api-platform/core:^2.1" if you know which you need.

...as the message said in the last sentence, I try it with:

composer require api-platform/core:*

...also not working. Message:

Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - symfony/flex is locked to version v1.6.2 and an update of this package was not requested.
    - symfony/flex v1.6.2 requires composer-plugin-api ^1.0 -> found composer-plugin-api[2.3.0] but it does not match the constraint.

Next I try it with the option "--with-all-dependencies" which was also recommended in the error message before. So I run:

composer require api-platform/core:* --with-all-dependencies

... it ends up with the same error message as before.

I take a look back to the Symfonycast tutorial. On the page it the possibility to see the composer.json which was used in the tutorial. I see that the symfony version was the same as my "4.2" the api-platform/core was of version "^2.1". So I tried the command with the version "^2.1":

composer require api-platform/core:^2.1 --with-all-dependencies

... nope, not working, error message:

Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - symfony/flex is locked to version v1.6.2 and an update of this package was not requested.
    - symfony/flex v1.6.2 requires composer-plugin-api ^1.0 -> found composer-plugin-api[2.3.0] but it does not match the constraint.
  Problem 2
    - ocramius/package-versions 1.4.2 requires composer-plugin-api ^1.0.0 -> found composer-plugin-api[2.3.0] but it does not match the constraint.
    - doctrine/doctrine-migrations-bundle 2.1.2 requires doctrine/migrations ^2.2 -> satisfiable by doctrine/migrations[2.2.1].
    - symfony/orm-pack v1.0.8 requires doctrine/doctrine-migrations-bundle * -> satisfiable by doctrine/doctrine-migrations-bundle[2.1.2].
    - doctrine/migrations 2.2.1 requires ocramius/package-versions ^1.3 -> satisfiable by ocramius/package-versions[1.4.2].
    - symfony/orm-pack is locked to version v1.0.8 and an update of this package was not requested.

That's frustrating :/ ...

Next what I was going to do was to clear the cache of composer, in hope that that's the problem. I run

composer clearcache
That's done without any error message and then run again:

composer require api-platform/core:^2.1 --with-all-dependencies

... nope that don't make a different...

Okay, next thing I've done was to delete my composer.lock and my vendor-folder and do a complete install of the dependencies with:

composer install

Next error. This time after composer installed all dependencies and then try to clear the cache. Message:

Script cache:clear returned with error code 1
!!
!!  In ArrayNode.php line 331:
!!  

!!    Unrecognized options "dir_name, namespace" under "doctrine_migrations". Available options are "all_or_nothing", "check_database_platform", "connection
"
!!    , "custom_template", "em", "factories", "migrations", "migrations_paths", "organize_migrations", "services", "storage".

!!  

!!
!!
Script @auto-scripts was called via post-update-cmd

After a short googleing it seems that this error comes from the doctrine/doctrine-migrations-bundle dependency which should be use in version 2.0 but installed was 3.0.
So I changed the corresponding entry of this package in the composer.json to version 2.0 and run:

composer update --with-all-dependencies

...and surprise done without an error message.
Okay new hope for my actual problem.
As a reminder I want to install the api-platform with composer...

So I run again:

composer require api-platform/core:^2.1 --with-all-dependencies

... and YES it installed the api-platfrom dependency without an error messages. I hadn't believed in it anymore after this chain of error (pain).

Supplement:
As I continued with the Tutorial from Symfonycast I saw that with the origin command

composer require api:1.2.0

they installed not only just the api-platform/core bundle but also some other bundles with seems to be helpful.

They installes a package with called api-platform/api-pack which contains the following child bundles:

• api-platform/core
• nelmio/cors-bundle
• symfony/asset
• symfony/expression-language
• symfony/orm-pack
• symfony/security-bundle
• symfony/serializer-pack
• symfony/twig-bundle
• symfony/validator

I think that I also need those bundles later on. So back in the "rabbit hole of dependencies" I run:

composer require api-platform/api-pack:1.2.2 --with-all-dependencies

...runs and ...also successful, puhhh yeah.

I used the version 1.2.2 of the api-pack, because this was the latest version in which the api-platfrom/core:2.1 was included.
Those Informations about the composer bundles you can find under:

Packagist

See you next time in the dependency hell...