This is a classic simple programming question. There are several ways to approach this problem. Here is one

solution:

The comparison function:

<?php
function is_anagram($a, $b)
{
    if (strlen($a) != strlen($b)) {
        // if not same size then they definitely aren't
        return false;
    }
// turn 'foobar' into ['f', 'o', 'o', 'b', 'a', 'r']
$a_chars = str_split($a);
$b_chars = str_split($b);
// sort them...
sort($a_chars);
sort($b_chars);
// check if they're exactly the same...
return $a_chars === $b_chars;
// another way to do this that someone emailed me about, which is much more simple:
// return (count_chars($a, 1) == count_chars($b, 1))
}

Let's write some tests

<?php
$anagrams = [
    // $a, $b, $is_valid_anagram
        ["asdf", "asdf", true],
        ["asdf", "fdsa", true],
        ["asdfasdf", "asdfasdf", true],
        ["a", "a", true],
        ["a", "b", false],
        ["aaa", "aaa", true],
        ["aaa", "aa", false],
        ["aaaa", "abab", false],
        ["qwerty", "asdfg", false]
];

And now let's go through each one.

<?php
foreach ($anagrams as $anagram) {
    [$a, $b, $is_valid_anagram] = $anagram;
    $result = is_anagram($a, $b);
    $result_message = $result ? "Is an anagram" : "Is not an anagram";
    if ($result != $is_valid_anagram) {
        throw new \Exception("The anagram function returned an incorrect result for $a and $b");  // this doesn't happen with our function! :)
    }
    var_dump("[$a] and [$b] $result_message");
}

"Our function said that [asdf] and [asdf] Is an anagram, which is correct"

"Our function said that [asdf] and [fdsa] Is an anagram, which is correct"

"Our function said that [asdfasdf] and [asdfasdf] Is an anagram, which is correct"

"Our function said that [a] and [a] Is an anagram, which is correct"

"Our function said that [a] and [b] Is not an anagram, which is correct"

"Our function said that [aaa] and [aaa] Is an anagram, which is correct"

"Our function said that [aaa] and [aa] Is not an anagram, which is correct"

"Our function said that [aaaa] and [abab] Is not an anagram, which is correct"

"Our function said that [qwerty] and [asdfg] Is not an anagram, which is correct"

BTW, not sure about how the [$a, $b, $is_valid_anagram] = $anagram; line works? See my blog post about array destructuring in PHP 7.