unable to get form values posted to mySQL db (phpMyAdmin 3.1.5)

I have php code that is inserting values from a form into my database. It is successfully creating the record in the database, but it is not taking the values for three of the 4 fields.

$sql=“INSERT INTO $tbl_name(name, email, comment, datetime)VALUES(’$name’, ‘$email’, ‘$comment’, ‘$datetime’)”;

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