- Agglomerative Hierarchical clustering method allows the clusters to be read from bottom to top and it follows this approach so that the program always reads from the sub-component first then moves to the parent. Whereas, divisive uses top-bottom approach in which the parent is visited first then the child. 

- Agglomerative hierarchical method consists of objects in which each object creates its own clusters and these clusters are grouped together to create a large cluster. It defines a process of merging that carries on till all the single clusters are merged together into a complete big cluster that will consists of all the objects of child clusters. Whereas, in divisive the parent cluster is divided into smaller cluster and it keeps on dividing till each cluster has a single object to represent.

There are many algorithms that can be used to analyze the database to check the maintenance of all the data sets that are already present. The different types of cluster models include as follows:

- Connectivity models: these are the models that connect one cluster to another cluster. This includes the example of hierarchical clustering that is based on the distance connectivity of one model to another model. 

- Centroid models: these are the models that are used to find the clusters using the single mean vector. It includes the example of k-means algorithm.

- Distribution models: it includes the specification of the models that are statistically distributed for example multivariate normal distribution model.

- Density models: deals with the clusters that are densely connected with one another in the regions having the data space. 

- Group models: specifies the model that doesn’t provide the refined model for the output and just gives the grouping information

Cluster analysis is used to define the object without giving the class label. It analyzes all the data that is present in the data warehouse and compare the cluster with the cluster that is already running. It performs the task of assigning some set of objects into the groups are also known as clusters. It is used to perform the data mining job using the technique like statistical data analysis. It includes all the information and knowledge around many fields like machine learning, pattern recognition, image analysis and bio-informatics. Cluster analysis performs the iterative process of knowledge discovery and includes trials and failures. It is used with the pre-processing and other parameters as a result to achieve the properties that are desired to be used.

Star schema: A highly de-normalized technique. A star schema has one fact table and is associated with numerous dimensions table and depicts a star.

Snow flake schema: The normalized principles applied star schema is known as Snow flake schema. Every dimension table is associated with sub dimension table.

Differences:

- A dimension table will not have parent table in star schema, whereas snow flake schemas have one or more parent tables.

- The dimensional table itself consists of hierarchies of dimensions in star schema, where as hierarchies are split into different tables in snow flake schema. The drilling down data from top most hierarchies to the lowermost hierarchies can be done.

OLTP stands for OnLine Transaction Processing. Applications that supports and manges transactions which involve high volumes of data are supported by OLTP system. OLTP is based on client-server architecture and supports transactions across networks.

OLAP stands for OnLine Analytical Processing. Business data analysis and complex calculations on low volumes of data are performed by OLAP. An insight of data coming from various resources can be gained by a user with the support of OLAP.

Both GET and POST are used to collect data from a form. However, when security is desired $_POST should be used. When the $_ POST variable is used, all variables used are NOT displayed in the URL. Also, they have no restrictions to the length of the variables. GET should be used when the interaction with the form is more like a question. For e.g. firing a query etc. POST should be used more often when the interaction is more like an order or the interaction changes the state of the resource.

POST example:

<form action="sample.php" method="post">

Name: <input type="text" name="name" />

Age: <input type="text" name="age" />

<input type="submit" />

</form>

On clicking submit the URL resembles to: https://www.mysamplesite.com/sample.php

The sample.php file can be used for catching the form data using $_POST 

Hello <?php echo $_POST["name"]; ?>.<br />

You are <?php echo $_POST["age"]; ?> years old!

GET EXAMPLE

<form action="sample.php" method="get">

Name: <input type="text" name="name" />

Age: <input type="text" name="age" />

<input type="submit" />

</form>

On clicking submit the URL resembles to : https://www.mysamplesite.com/sample.php?name=jim&age=37

The sample.php file can be used for catching the form data using $_GET 

Hello <?php echo $_GET["name"]; ?>.<br />

You are <?php echo $_GET["age"]; ?> years old!

A HTML form should be build before uploading the file. The following HTML code is used for selecting the file that is to be uploaded. The type attribute of input must be “file”.

<form enctype="multipart/form-data" action="uploader.php" method="POST">

<input type="hidden" name="MAX_FILE_SIZE" value="100000" />

Choose a file to upload: <input name="uploadedfile" type="file" /><br />

<input type="submit" value="Upload File" />

</form>

At the time of executing uploader.php, the uploaded file will be stored in a temporary storage are on the webserver. An associative array $_FILES['uploadedfile']['name'] is used for uploading. The ‘name’ is the original file that is to be uploaded. Another associative array $_FILES['uploadedfile']['tmp_name'] is used for placing the uploaded file in a temporary location on the server and the file should be empty and should exist with the tmp_name.

The following code snippet is used for uploading the file.

$target_path = "uploads/"; // the target location of the file

$target_path = $target_path . basename( $_FILES['uploadedfile']['name']);

if(move_uploaded_file($_FILES['uploadedfile']['tmp_name'], $target_path)) 

{

       echo "The file ". basename( $_FILES['uploadedfile']['name']). 

        " has been uploaded";

}

else

{

      echo “There was an error while uploading the file”;

}

The function move_uploaded_file()is used to place the source file into the destination folder, which resides on the server.

If the uploading is successful, the message “The file filename has been uploaded. Otherwise the error message “There was an error while uploading the file” would be displayed.

HTTP headers can be used in PHP by redirection which is written as:

<?header('Location: https://www.php.net')?>

The headers can be added to HTTP response in PHP using the header(). The response headers are sent before any actual response being sent. The HTTP headers have to be sent before taking the output of any data. The statement above gets included at the top of the script.