src/ActiveQuery.php
public function scalar($attribute, $db = null)
    {
        if ($this->emulateExecution) {
            return null;
        }

        $record = $this->one($db);
        if ($record !== null) {
            return $record->hasAttribute($attribute) ? $record->$attribute : null;
        } else {
            return null;
        }
    }
public function scalar($attribute, $db = null)
    {
        if ($this->emulateExecution) {
            return null;
        }

        $record = $this->one($db);
        if ($record !== null) {
            return $record->hasAttribute($attribute) ? $record->$attribute : null;
        } else {
            return null;
        }
    }
Returns the query result as a scalar value.
The value returned will be the specified attribute in the first record of the query results.
@param string $attribute name of the attribute to select
@param Connection $db the database connection used to execute the query.
If this parameter is not given, the `db` application component will be used.
@return string the value of the specified attribute in the first record of the query result.
Null is returned if the query result is empty.