Ask: Questions about filter function

Ask: questions about filter functions
Hello everyone:
About Filter function I think the official information is not very clear, special to ask you:
Filter_sanitize_string This filter "remove tags, remove or encode special characters" specifically what special characters ah? Which special characters are removed, and which are encoded? Who has specific information about this?
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Mysql_real_escape
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References:

I want to filter the user's input, 1. Remove letters, numbers, and other characters from Chinese characters. 2. Anti-SQL injection. Is there an existing filter function available in PHP?

achieved 1, the implementation of 2.
Assume that you are using UTF-8 encoding
$text = preg_replace ('/\w/u ', ' ', $text);

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