Existential Quantification of Provable Arithmetic Formula is Provable

Theorem
Let $\map \phi x$ be a WFF in the language of arithmetic with $1$ free variable.

Let $\sqbrk a$ denote the unary representation of $a$.

Suppose that, for every $n \in \N$ such that $\N \models \map \phi {x \gets \sqbrk n}$:
 * $\map \phi {x \gets \sqbrk n}$

is a theorem of minimal arithmetic.

Suppose also that:
 * $\N \models \exists x: \map \phi x$

Then:
 * $\exists x: \map \phi x$

is a theorem of minimal arithmetic.

Proof
By definition of value of formula:
 * $\exists x \in \N: \map \phi x$

Choose $x_0 \in \N$ that satisfies $\map \phi {x \gets x_0}$.

By Unary Representation of Natural Number:
 * $x_0 = \sqbrk {x_0}$

Thus, by Substitution Property of Equality:
 * $\map \phi {x \gets \sqbrk {x_0} }$

But then, by hypothesis:
 * $\map \phi {x \gets \sqbrk {x_0} }$

is a theorem of minimal arithmetic.

The result follows from Existential Generalisation.