Two events $A$ and $B$ are independent (noted $A \perp B$) if $P( A \cap B) = P(A)P(B)$, that's the definition of independence for two events.
The definition of $P(A \mid B)$ is
$$
P(A \mid B) = \frac{P(A \cap B)}{P(B)}
$$
Then
\begin{align*}
A \perp B &\iff P(A \cap B) = P(A)P(B) \\
&\iff \frac{P(A \cap B)}{P(B)} = \frac{P(A) P(B)}{P(B)} = P(A) \\
&\iff \frac{P(A \cap B)}{P(A)} = \frac{P(A) P(B)}{P(A)} = P(B) \\
&\iff P(A \mid B) = P(A) \\
&\iff P(B \mid A) = P(B)
\end{align*}
Thus if $P(A \mid B) = P(A)$ then $A$ and $B$ are independent and you will also have $P(B \mid A) = P(B)$.
To answer your first question : if one holds the other one holds too.
This can be found on Wikipedia here.