mysql_real_escape_string() needs an active connection to the MySQL server and will initiate one with the default data from the php.ini configuration if there is none.
Do not use this function without first connecting to the database.
Also, do not use the mysql_* functions. They are deprecated and will be removed from PHP.
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solved Warning: mysql_real_escape_string() [function.mysql-real-escape-string]: A link to the server could not be established in [duplicate]