Help with Function

I was given this function question for an interview. I don't think I'm interested in the job and therefore will not submit this, but I am curious how to answer this. Once I start something like this, I can't just quit without knowing the answer.

Questions:
1. Does the function have any errors?
2. What is the purpose of the function?
3. What can be done to improve it?
4. Why is isset() used instead of a simple conditional used above it?
5. What assumptions can you make about the database structure?

function auto_query( $table, $vars ) {
	$_result = $this->query( "EXPLAIN $_table_name" );
	$_fields = array(); // Empty array assigned to variable
	
	while( $_row = $_result->fetchRow() ) {
		array_push( $_fields, $_row['Field'] );  // Push one or more elements onto the end of array
	}
	
	if( $vars[$table . 'id'] ) {
		$_query = "UPDATE $_table SET ";
	} else {
		$_query = "INSERT INTO $_table SET " . $_table . '_date_created = NOW(), ';
	}
	
	$_query_params = array();  // Empty array assigned to variable
	
	foreach( $_fields as $_field ) {
		if( isset( $_vars[$_field] ) ) {
			$_query_params[] = "$_field = " . $this->dbh->quoteSmart( $_vars[$_field] );
		}
	}
	
	$_query .= implode( ',', $_query_params );  // Join array elements with a string
	
	if( $_vars[$_table . '_id'] ) {
		$_query .= " WHERE {$_table}_id = " . $this->dbh->quoteSmart($_vars[$_table . '_id']);
	}
	
	return $_query;
}