$input = "\\'"; # these two backslashes will print a single backslash in the SQL, but, as I will add one backslash before the single quote, the result there will be just the same as you can see here.

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select username, password from users where sid='$input';

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5' or 1='1

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5\\' or 1=\\'1

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...where sid='    5\\' or 1=\\'1        ';

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\\