File Reading

Since PHP is a server side technology it is allowed to create, read, and write files on the server's filesystem.

To open a file for reading one can do:

$fileHandle = fopen("my.dat", "r"); 
$file_string = fread($fileHandle, filesize("my.dat"));
fclose($fileHandle);

Here fread reads in its second parameter many bytes.

To read in a single line from a file one can use:

$line = fgets($fileHandle, $max_num_bytes_line);

Alternatively, one can read the whole file in as a string using a single command like:
```
$string = file_get_contents("my.dat");
```
Similarly, sometimes it is useful to read in the whole file as an array of lines:
```
$lines = file("my.dat"); 
```

File Writing

File writing can be done in a similar fashion to file reading in PHP.

$fileHandle = fopen("my.dat", "w");  // use "a" for append
fwrite($fileHandle, $out_data);
fclose($fileHandle);

One can also write out a whole file based on a string using:
```
file_put_contents("out.dat", $str);
```

File Locking

Unless you as a coder do something, it is completely possible for two scripts to try to access the same file at the same time.

To prevent this you should call the flock function to get a lock before you try to do something with a file:

$fp = fopen("/tmp/lock.txt", "w");
if (flock($fp, LOCK_EX)) { 
   // do an exclusive/write lock. use LOCK_SH (for  shared/read lock)
   fwrite($fp, "Write something here\n");
   flock($fp, LOCK_UN); // release the lock
} else {
   echo "Couldn't lock the file !";
}
fclose($fp);

Locks are released when fclose() is called.

Permissions and Unix-like File Manipulation

PHP supports several functions for manipulating files and directories:

rename($oldname, $newname); //like Unix mv command, renames/moves a file
copy($source, $dest); //copies a file, like Unix cp
unlink($filename); // delete a file
mkdir($path); // make a directory
rmdir($path); // delete a directory
link($target, $name); // create a link
chmod($filename, $mode); // change file permissions
chown($filename, $user); // change owner of a file or dir or link
chgrp($filename, $group); //change group of a file or dir or link
stat($filename); 
   //returns an array saying size, last access, last modified, etc.  for a file
touch($filename [, int $time = time() [, int $atime ]]); 
   //sets modification/ access times of a file

Directory Handling in PHP

You can read the contents of a directory (aka a folder) with the following PHP code:

if(file_exists('/path/to/folder')) { //file_exists check if a file or dir exists
    $h = opendir('/path/to/folder'); //can use is_dir to check is something is a directory
    while (($item = readdir($h)) !== false) {
        $type = is_dir($item) ? "directory" : 
            (is_link($item) ? "link" : 
            "file");
        //there is also a is_file
        echo "$item is a $type\n";
    }
}

Alternatively, you can get an array of all file names whose path matches a file pattern using the glob command:
```
$text_files = glob("/some_path/*.txt");
foreach($text_files as $file_name) {
   //do something
}
```

Cookies

Sometimes it is useful to remember a client when it comes back, To do this one can use the HTTP-Cookie protocol.
The Server can send as one of its response headers: Set-Cookie: name=value; expires=some date; path=some path; domain=some_domain;
In PHP you can write out a header using the header() function.
When the Client comes back, it will send the cookie as part of its request header as: Cookie: name=value

Cookies in PHP

Cookies can be set using the setcookie function:
setcookie(name, value, expires);
This should be done before output is produced by your script.
To access the value of a cookie returned from a browser you can use the $_COOKIE array.

Quiz

Which of the following statements is true?

PHP can only be run as a Web Server module.
PHP's $_REQUEST variable is an associative array that gets automatically set up for you with the contents of both the POST'd or GET'd form variables.
Local variables in a PHP function can be initialized using the local keyword.

Sessions

A session is a collection of information about one user's interaction with a web site.
It is usually transient in nature and has some lifetime associated with it.
For example, interactions with a web-site leading up to a purchase. If a purchase is eventually made, the purchase data is permanently records in the database. The permanent store being different from the session.
Cookies can be used to implement sessions. A single name-value pair: session_id=some-random-string is sent as a cookie together with an expiry time. This pair when sent from the browser is used to look up on the server a file containing a serialized associative array of user data.
PHP supports session management.
To start a session one calls session_start(); . This is what sends the Set-Cookie header so needs to before any html is output. The default name used for the session_id is PHPSESSID. YOu can change this with the function session_name($some_name);

Then to set/get values of the session one uses the global array variable $_SESSION:

$_SESSION["test"]=37; /* sets the test session variable*/
echo $_SESSION["test"];

Database Management Systems and Databases

For our purposes a database management systems (DBMS), is used to manage a collection of databases.
That is, it manages how the data is stored in each database, and who can access it when.
A given database consists of a collection of tables.
Each table has a type for each of its columns and then has rows of data where the data values match the values of the column types.
Example: Might have a table car(make VARCHAR(10), model VARCHAR(10), year DATE). A possible row could be
```
('Toyota', 'Corolla', '1970-01-01')
```

MySQL Databases

To manage MySQL databases you need to know how to:
- Create and manage users
- Create new databases and tables
- Do lots of other stuff I won't talk about
- Many of these operations can be done using phpMyAdmin.
- Alternatively, you can use the command line.

PHP: Files, Cookies, Sessions, Databases

Outline