The error Illegal string offset 'whatever' in... generally means: you're trying to use a string as a full array.

That is actually possible since strings are able to be treated as arrays of single characters in php. So you're thinking the $var is an array with a key, but it's just a string with standard numeric keys, for example:

$fruit_counts = array('apples'=>2, 'oranges'=>5, 'pears'=>0);
echo $fruit_counts['oranges']; // echoes 5
$fruit_counts = "an unexpected string assignment";
echo $fruit_counts['oranges']; // causes illegal string offset error

You can see this in action here:

For those who come to this question trying to translate the vagueness of the error into something to do about it, as I was.

You're trying to access a string as if it were an array, with a key that's a string. string will not understand that. In code we can see the problem:

"hello"["hello"];
// PHP Warning:  Illegal string offset 'hello' in php shell code on line 1

"hello"[0];
// No errors.

array("hello" => "val")["hello"];
// No errors. This is *probably* what you wanted.

In depth

Let's see that error:

Warning: Illegal string offset 'port' in ...

What does it say? It says we're trying to use the string 'port' as an offset for a string. Like this:

$a_string = "string";

// This is ok:
echo $a_string[0]; // s
echo $a_string[1]; // t
echo $a_string[2]; // r
// ...

// !! Not good:
echo $a_string['port'];
// !! Warning: Illegal string offset 'port' in ...