reorg and add code for second edition

09_dijkstras_algorithm/php/01_dijkstras_algorithm.php

@@ -0,0 +1,73 @@
+<?php
+$graph = [];
+$graph["start"] = [];
+$graph["start"]["a"] = 6;
+$graph["start"]["b"] = 2;
+
+$graph["a"] = [];
+$graph["a"]["fin"] = 1;
+
+$graph["b"] = [];
+$graph["b"]["a"] = 3;
+$graph["b"]["fin"] = 5;
+
+$graph["fin"] = [];
+
+# the costs table
+$infinity = PHP_INT_MAX;
+$costs = [];
+$costs["a"] = 6;
+$costs["b"] = 2;
+$costs["fin"] = $infinity;
+
+# the parents table
+$parents = [];
+$parents["a"] = "start";
+$parents["b"] = "start";
+$parents["fin"] = null;
+
+$processed = [];
+
+function findLowestCodeNode(array $costs) {
+    $lowestCost = PHP_INT_MAX;
+    $lowestCostNode = null;
+    global $processed;
+    # Go through each node.
+    foreach ($costs as $node => $cost) {
+        # If it's the lowest cost so far and hasn't been processed yet...
+        if ($cost < $lowestCost && !array_key_exists($node, $processed)) {
+            # ... set it as the new lowest-cost node.
+            $lowestCost = $cost;
+            $lowestCostNode = $node;
+        }
+    }
+
+    return $lowestCostNode;
+}
+
+# Find the lowest-cost node that you haven't processed yet.
+$node = findLowestCodeNode($costs);
+
+# If you've processed all the nodes, this while loop is done.
+while ($node) {
+    $cost = $costs[$node];
+    # Go through all the neighbors of this node.
+    $neighbors = $graph[$node];
+    foreach (array_keys($neighbors) as $n) {
+        $newCost = $cost + $neighbors[$n];
+        # If it's cheaper to get to this neighbor by going through this node...
+        if ($costs[$n] > $newCost) {
+            # ... update the cost for this node.
+            $costs[$n] = $newCost;
+            # This node becomes the new parent for this neighbor.
+            $parents[$n] = $node;
+        }
+    }
+    # Mark the node as processed.
+    $processed[$node] = true;
+    # Find the next node to process, and loop.
+    $node = findLowestCodeNode($costs);
+}
+
+print("Cost from the start to each node:");
+var_dump($costs);