wpdb->get_row is selecting the variable as a column name

The error in question happened because you did not properly build your SQL query/statement:

The column name in your query is hashcode, and you are trying to get the row where the column value matches the value of the $file_hash variable, i.e. WHERE {$file_hash} = `hashcode`.
But because the value is a string/text (and not a number), then it needs to be wrapped in quotes like so: '$file_hash'.
And if you don’t do that, then the value would be seen as the column name instead of the supposedly column value.

So fixing the issue is basically easy — just wrap the variable with quotes like so:

SELECT *
FROM {$wpdb->prefix}vip_files 
WHERE '$file_hash' = `hashcode`

However, that query is highly prone to SQL injection attacks because the value of $file_hash is not escaped, so you need to escape it, and I would suggest you to use wpdb::prepare() like so:

$query = $wpdb->prepare( "SELECT *
    FROM {$wpdb->prefix}vip_files 
    WHERE %s = `hashcode`",
    $file_hash
);

$file = $wpdb->get_row( $query );

Also, remember that wpdb::prepare() requires at least two parameters: the SQL query with placeholders like %s; and the replacement values for the placeholders used in that SQL query, like so: $wpdb->prepare( "SELECT * FROM table_name WHERE some_column = %s", 'column value' ).

So this is actually incorrect: ^{(this is what you used as you said in the comments)}

$wpdb->prepare("SELECT * FROM {$wpdb->prefix}vip_files WHERE '{$file_hash}' = `hashcode`")

More specifically, yes, that code does work in that MySQL would not throw any error. But it will result in a PHP notice saying “wpdb::prepare was called incorrectly“. And what’s worse is, the above code will not actually going to escape the $file_hash variable!
And the correct code is this — I used the %s placeholder and the $file_hash is now the second parameter for $wpdb->prepare():

$wpdb->prepare("SELECT * FROM {$wpdb->prefix}vip_files WHERE %s = `hashcode`", $file_hash)

So make sure to properly call wpdb::prepare() and note that escaping the variable value not only ensures the query to be safe from SQL injections, but escaping would also help us avoid syntax errors in SQL. I mean, when the variable value is something like 90's favorites (note the quote).

And I hope that helps, and do check out the full class reference for more details about the class and its methods/functions and properties. 🙂

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