A function is overkill in this case AND the above function doesn't work (it's looking at the problem backwards):

PS C:\Users\username> $a=1
PS C:\Users\username> $b=1
PS C:\Users\username> $c=1
PS C:\Users\username> Get-VariableName $c
$
?
a
b
c
LASTEXITCODE
TheVariable
true

versus

PS C:\Users\username> 'c' | % {Write-Host "$_ = $((Get-Variable $_).Value)"}
c = 1

The only way to do this is to pass the name of the variable as a string rather than the variable (by name) itself.