Update a record on another table and get an string error // MySQL

I use a button as typ “php” to update records on another table. One field is a varchar-typ in the database of MySQL. After pressinng the button I get an error: “Array to string conversion”.


$update_table = ‘test’;
$update_where = "test ";
$update_fields = array(
“Einheit = ‘{Einheit}’”, // it throws an error
);

$update_sql = ‘UPDATE ’ . $update_table
. ’ SET ’ . implode(’, ', $update_fields)
. ’ WHERE ’ . $update_where;
sc_exec_sql($update_sql);


I tried also
"Einheit = ’ “.{Einheit}.” ’ ";

Any suggestions. Thanks.