Yii2 user data verification problems

Write rules in the rules method of a model, {code ...} it should be said that the content should be filtered like the trim function. In fact, the error "ReflectionExceptionClassstrip_tagsdoesnotexist" is reported why trim is used as a function, but strip_tags is used as a class... write rules in the rules method of a model,

public function rules()    {        return [            [['title','description','keywords'],'strip_tags']        ];    }

It is reasonable to say that the trim function is the same as the trim function to filter the content, but an error is actually returned.
ReflectionException
Class strip_tags does not exist
Why is trim treated as a function but strip_tags treated as a class?

Reply content:

Write rules in the rules method of a model,

public function rules()    {        return [            [['title','description','keywords'],'strip_tags']        ];    }

It is reasonable to say that the trim function is the same as the trim function to filter the content, but an error is actually returned.
ReflectionException
Class strip_tags does not exist
Why is trim treated as a function but strip_tags treated as a class?

NottrimAs a function.

    public static $builtInValidators = [        'boolean' => 'yii\validators\BooleanValidator',        'captcha' => 'yii\captcha\CaptchaValidator',        'compare' => 'yii\validators\CompareValidator',        'date' => 'yii\validators\DateValidator',        'default' => 'yii\validators\DefaultValueValidator',        'double' => 'yii\validators\NumberValidator',        'each' => 'yii\validators\EachValidator',        'email' => 'yii\validators\EmailValidator',        'exist' => 'yii\validators\ExistValidator',        'file' => 'yii\validators\FileValidator',        'filter' => 'yii\validators\FilterValidator',        'image' => 'yii\validators\ImageValidator',        'in' => 'yii\validators\RangeValidator',        'integer' => [            'class' => 'yii\validators\NumberValidator',            'integerOnly' => true,        ],        'match' => 'yii\validators\RegularExpressionValidator',        'number' => 'yii\validators\NumberValidator',        'required' => 'yii\validators\RequiredValidator',        'safe' => 'yii\validators\SafeValidator',        'string' => 'yii\validators\StringValidator',        'trim' => [            'class' => 'yii\validators\FilterValidator',            'filter' => 'trim',            'skipOnArray' => true,        ],        'unique' => 'yii\validators\UniqueValidator',        'url' => 'yii\validators\UrlValidator',        'ip' => 'yii\validators\IpValidator',    ];

Parent class yii/base/Model: createValidators. You can refer to this method to generate the validator method.
Traverse rules and Judge $ rule [1], which is equivalent to the fields 'trim' and 'strip _ tags '.
1. Whether it is an anonymous Function（$rule instanceof \Closure）Or is it a subclass method?
2. determine whether an existing validator exists.

public static $builtInValidators = [        'boolean' => 'yii\validators\BooleanValidator',        'captcha' => 'yii\captcha\CaptchaValidator',        'compare' => 'yii\validators\CompareValidator',        'date' => 'yii\validators\DateValidator',        'default' => 'yii\validators\DefaultValueValidator',        'double' => 'yii\validators\NumberValidator',        'each' => 'yii\validators\EachValidator',        'email' => 'yii\validators\EmailValidator',        'exist' => 'yii\validators\ExistValidator',        'file' => 'yii\validators\FileValidator',        'filter' => 'yii\validators\FilterValidator',        'image' => 'yii\validators\ImageValidator',        'in' => 'yii\validators\RangeValidator',        'integer' => [            'class' => 'yii\validators\NumberValidator',            'integerOnly' => true,        ],        'match' => 'yii\validators\RegularExpressionValidator',        'number' => 'yii\validators\NumberValidator',        'required' => 'yii\validators\RequiredValidator',        'safe' => 'yii\validators\SafeValidator',        'string' => 'yii\validators\StringValidator',        'trim' => [            'class' => 'yii\validators\FilterValidator',            'filter' => 'trim',            'skipOnArray' => true,        ],        'unique' => 'yii\validators\UniqueValidator',        'url' => 'yii\validators\UrlValidator',    ];