Basic one dimensional arrays
$array = array(3, 5, 2, 8);
Applicable sort functions:
sortrsortasortarsortnatsortnatcasesortksortkrsort
The difference between those is merely whether key-value associations are kept (the "a" functions), whether it sorts low-to-high or reverse ("r"), whether it sorts values or keys ("k") and how it compares values ("nat" vs. normal). See http://php.net/manual/en/array.sorting.php for an overview and links to further details.
Multi dimensional arrays, including arrays of objects
$array = array(
array('foo' => 'bar', 'baz' => 42),
array('foo' => ..., 'baz' => ...),
...
);
If you want to sort $array by the key 'foo' of each entry, you need a custom comparison function. The above sort and related functions work on simple values that they know how to compare and sort. PHP does not simply "know" what to do with a complex value like array('foo' => 'bar', 'baz' => 42) though; so you need to tell it.
To do that, you need to create a comparison function. That function takes two elements and must return 0 if these elements are considered equal, a value lower than 0 if the first value is lower and a value higher than 0 if the first value is higher. That's all that's needed:
function cmp(array $a, array $b) {
if ($a['foo'] < $b['foo']) {
return -1;
} else if ($a['foo'] > $b['foo']) {
return 1;
} else {
return 0;
}
}
Often, you will want to use an anonymous function as the callback. If you want to use a method or static method, see the other ways of specifying a callback in PHP.
You then use one of these functions:
Again, they only differ in whether they keep key-value associations and sort by values or keys. Read their documentation for details.
Example usage:
usort($array, 'cmp');
usort will take two items from the array and call your cmp function with them. So cmp() will be called with $a as array('foo' => 'bar', 'baz' => 42) and $b as another array('foo' => ..., 'baz' => ...). The function then returns to usort which of the values was larger or whether they were equal. usort repeats this process passing different values for $a and $b until the array is sorted. The cmp function will be called many times, at least as many times as there are values in $array, with different combinations of values for $a and $b every time.
To get used to this idea, try this:
function cmp($a, $b) {
echo 'cmp called with $a:', PHP_EOL;
var_dump($a);
echo 'and $b:', PHP_EOL;
var_dump($b);
}
All you did was define a custom way to compare two items, that's all you need. That works with all sorts of values.
By the way, this works on any value, the values don't have to be complex arrays. If you have a custom comparison you want to do, you can do it on a simple array of numbers too.
sort sorts by reference and does not return anything useful!
Note that the array sorts in place, you do not need to assign the return value to anything. $array = sort($array) will replace the array with true, not with a sorted array. Just sort($array); works.
Custom numeric comparisons
If you want to sort by the baz key, which is numeric, all you need to do is:
function cmp(array $a, array $b) {
return $a['baz'] - $b['baz'];
}
Thanks to The PoWEr oF MATH this returns a value < 0, 0 or > 0 depending on whether $a is lower than, equal to or larger than $b.
Note that this won't work well for float values, since they'll be reduced to an int and lose precision. Use explicit -1, 0 and 1 return values instead.
Objects
If you have an array of objects, it works the same way:
function cmp($a, $b) {
return $a->baz - $b->baz;
}
Functions
You can do anything you need inside a comparison function, including calling functions:
function cmp(array $a, array $b) {
return someFunction($a['baz']) - someFunction($b['baz']);
}
Strings
A shortcut for the first string comparison version:
function cmp(array $a, array $b) {
return strcmp($a['foo'], $b['foo']);
}
strcmp does exactly what's expected of cmp here, it returns -1, 0 or 1.
Spaceship operator
PHP 7 introduced the spaceship operator, which unifies and simplifies equal/smaller/larger than comparisons across types:
function cmp(array $a, array $b) {
return $a['foo'] <=> $b['foo'];
}
Sorting by multiple fields
If you want to sort primarily by foo, but if foo is equal for two elements sort by baz:
function cmp(array $a, array $b) {
if (($cmp = strcmp($a['foo'], $b['foo'])) !== 0) {
return $cmp;
} else {
return $a['baz'] - $b['baz'];
}
}
For those familiar, this is equivalent to an SQL query with ORDER BY foo, baz.
Also see this very neat shorthand version and how to create such a comparison function dynamically for an arbitrary number of keys.
Sorting into a manual, static order
If you want to sort elements into a "manual order" like "foo", "bar", "baz":
function cmp(array $a, array $b) {
static $order = array('foo', 'bar', 'baz');
return array_search($a['foo'], $order) - array_search($b['foo'], $order);
}
For all the above, if you're using PHP 5.3 or higher (and you really should), use anonymous functions for shorter code and to avoid having another global function floating around:
usort($array, function (array $a, array $b) { return $a['baz'] - $b['baz']; });
That's how simple sorting a complex multi-dimensional array can be. Again, just think in terms of teaching PHP how to tell which of two items is "greater"; let PHP do the actual sorting.
Also for all of the above, to switch between ascending and descending order simply swap the $a and $b arguments around. E.g.:
return $a['baz'] - $b['baz']; // ascending
return $b['baz'] - $a['baz']; // descending
Sorting one array based on another
And then there's the peculiar array_multisort, which lets you sort one array based on another:
$array1 = array( 4, 6, 1);
$array2 = array('a', 'b', 'c');
The expected result here would be:
$array2 = array('c', 'a', 'b'); // the sorted order of $array1
Use array_multisort to get there:
array_multisort($array1, $array2);
As of PHP 5.5.0 you can use array_column to extract a column from a multi dimensional array and sort the array on that column:
array_multisort(array_column($array, 'foo'), SORT_DESC, $array);
You can also sort on more than one column each in either direction:
array_multisort(array_column($array, 'foo'), SORT_DESC,
array_column($array, 'bar'), SORT_ASC,
$array);
As of PHP 7.0.0 you can also extract properties from an array of objects.
If you have more common cases, feel free to edit this answer.
