Fastest way to link words in PHP array

I have a sentence in a simple PHP string:

$myString = "This is a test.";

I want an array that contains all possible connections between two words, or two groups of words, as follows (whitespace as a delimiter):

$myArray = [
    ["This", "is"],
    ["This", "is a"],
    ["This", "is a test."],
    ["This is", "a"],
    ["This is", "a test."],
    ["This is a", "test."],
    ["is", "a"],
    ["is", "a test."],
    ["is a", "test."],
    ["a", "test."]
];

It must run as fast as possible, hopefully in a single scan of the input string.