Inserting the variable into the mysql table does not work

I'm trying to insert a variable into mysql table, but it does not do what i want (no error, it simply does not work)

$string = 'this is an example';
mysql_query("insert into tablename (columnname) values ????");

Anyone can help me how to insert $string into table tablename? I tried replacing ???? with ($string), (".$string."), $string, but all of them does not insert the $string variable into the database.

Thanks in advance!

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You can use \" (an escaped quotation mark) to include quotes inside your query, like so:

$string = 'this is an example';
mysql_query("insert into tablename (columnname) values (\"$string\")");