How to specify a validation rule in Yii2 which will be greater than or less than of a specific number or value?

It should be:

['x', 'compare', 'compareValue' => 100, 'operator' => '<'],

and

['x', 'compare', 'compareValue' => 100, 'operator' => '>='],

accordingly.

Read more in official docs.

You could also use the min attribute on number, or integer validators:

['age', 'integer', 'min' => 0],
['amount', 'number', 'min' => 0],

There is also a max option.

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