PEAR is a framework and repository for reusable PHP components. PEAR stands for PHP Extension and Application Repository. It contains all types of PHP code snippets and libraries. It also provides a command line interface to install “packages” automatically.

Below are different sorting methods – 

• sort(): Sort by value in ascending order.
• rsort(): Sort by value in decending order.
• ksort(): Sort by key in ascending order.
• krsort(): Sort by key in decending order.
• asort(): Sort by value in ascending order.
• arsort(): Sort by value in decending order.
• usort(): Sort value with custom comparison function

Example  of usort()
function customSort($a, $b) {
    return $a - $b; // Sort in ascending order
}

$myArray = [3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5];
usort($myArray, "customSort");
print_r($myArray);
 

In PHP, concrete classes and methods function similarly to other object-oriented languages. A concrete class is one that can be instantiated and all of its methods are defined. PHP, just like Java or C#, uses classes and objects as the fundamental building blocks for creating complex data structures, such as linked lists.
Here is an example of a PHP concrete class for a node in a linked list and a method to reverse a linked list:
 

class ListNode {
    public $val;
    public $next;

    public function __construct($val) {
        $this->val = $val;
        $this->next = null;
    }

    // Reverse the linked list
    public static function reverse($head) {
        $prev = null;
        $current = $head;
        $next = null;

        while ($current !== null) {
            $next = $current->next; // temporarily store the next node
            $current->next = $prev; // reverse the current node's pointer
            $prev = $current;       // move pointers one position forward
            $current = $next;
        }

        return $prev; // prev is the new head of the reversed list
    }
}

// Usage:
// Create a linked list: 1 -> 2 -> 3 -> 4 -> null
$head = new ListNode(1);
$head->next = new ListNode(2);
$head->next->next = new ListNode(3);
$head->next->next->next = new ListNode(4);

// Reverse the linked list
$reversedHead = ListNode::reverse($head);

// Print the reversed linked list
$current = $reversedHead;
while ($current !== null) {
    echo $current->val . ' ';
    $current = $current->next;
}
// Output should be: 4 3 2 1

In this PHP example:

• ListNode is a concrete class as it can be instantiated and contains an implementation for the constructor and the static reverse method.
• The reverse static method takes the head of a linked list as its argument and returns the new head of the list after reversal.

The usage section of the script creates a simple linked list with the values 1 through 4, reverses the list, and then prints out the values of the reversed list. The output of the provided PHP code should display the numbers in reverse order.