Is there a JavaScript equivalent of PHP’s “variable variables”?

There is no single solution for this. It is possible to access some global variables dynamically via window, but that doesn't work for variables local to a function. Global variables that do not become a property of window are variables defined with let and const, and classes.

There is almost always a better solution than using variable variables! Instead you should be looking at data structures and choose the right one for your problem.

If you have a fixed set of names, such as

// BAD - DON'T DO THIS!!!
var foo = 42;
var bar = 21;

var key = 'foo';
console.log(eval(key));

store those names/values as properties of an object and use bracket notation to look them up dynamically:

// GOOD
var obj = {
  foo: 42,
  bar: 21,
};

var key = 'foo';
console.log(obj[key]);

In ES2015+ it's even easier to do this for existing variables using concise property notation:

// GOOD
var foo = 42;
var bar = 21;
var obj = {foo, bar};

var key = 'foo';
console.log(obj[key]);

If you have "consecutively" numbered variables, such as

// BAD - DON'T DO THIS!!!
var foo1 = 'foo';
var foo2 = 'bar';
var foo3 = 'baz';

var index = 1;
console.log(eval('foo' + index));

then you should be using an array instead and simply use the index to access the corresponding value:

// GOOD
var foos = ['foo', 'bar', 'baz'];
var index = 1;
console.log(foos[index - 1]);