在PHP中，对海量数据进行基数统计通常可以使用布隆过滤器（Bloom Filter）或者Count-Min Sketch算法。以下是使用Count-Min Sketch算法的一个简单示例：

class CountMinSketch {
    private $rows;
    private $columns;
    private $values;
 
    public function __construct($rows, $columns) {
        $this->rows = $rows;
        $rows = $rows + 1;
        $this->columns = $columns;
        $this->values = array_fill(0, $rows, array_fill(0, $columns, 0));
    }
 
    public function increment($item, $count) {
        $hashes = $this->generateHashes($item);
        foreach ($hashes as $hash) {
            $row = $hash % $this->rows;
            $column = ($hash >> $this->rows) % $this->columns;
            if ($this->values[$row][$column] > $count) {
                $this->values[$row][$column] = $count;
            }
        }
    }
 
    public function estimate($item) {
        $min = PHP_INT_MAX;
        $hashes = $this->generateHashes($item);
        foreach ($hashes as $hash) {
            $row = $hash % $this->rows;
            $column = ($hash >> $this->rows) % $this->columns;
            $min = min($min, $this->values[$row][$column]);
        }
        return $min;
    }
 
    private function generateHashes($item) {
        $hashes = array(
            hash("fnv1a32", $item) // FNV-1a 32-bit hash
        );
        // For better estimation, you can add more hash functions
        // e.g., MD5, SHA1, or a custom hash function
        // $hashes[] = hash("md5", $item);
        // $hashes[] = hash("sha1", $item);
        return $hashes;
    }
}
 
// 使用示例
$sketch = new CountMinSketch(1024, 2048); // 调整行和列的大小
$sketch->increment("item1", 1);
$sketch->increment("item2", 2);
 
echo "Estimated count for item1: " . $sketch->estimate("item1") . "\n";
echo "Estimated count for item2: " . $sketch->estimate("item2") . "\n";