Package phpunit/phpunit-mock-objects is abandoned, you should avoid using it. No replacement was suggested.

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4.5:

Package phpunit/phpunit-mock-objects is abandoned, you should avoid using it. No replacement was suggested.

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Hello,
Please help me to solve this error, I trying installing Silverstripe using composer.

It’s not an error, just a warning, and you should be able to just ignore it. That package should only be getting installed on a dev environment (which happens by default on local Silverstripe CMS installs) and only used when running tests.