how to do special character validation in PHP

I have lot of textarea fields to add comments and quotes. How can I 开发者_高级运维validate these fields. I checked null validation other than null validation what validation I have to do?

Some things you need to check :

• empty values

• numbers only

• input length

• email address

• strip html tags

Credit : php-mysql-tutorial.com

!empty( $text ) && strlen( $text ) > 123 && !preg_match('/(<[a-z]+>)/', $text )

... and so on...