More on PHP Arrays

Last Wednesday, we saw the basics of creating/accessing an array in PHP:
```
$arr=array(1,2,3); echo $arr[2]; /* would print 3 */
```

Recall also

$carr = array(); //creates an empty array

Arrays in PHP are similar to Perl hashes.
The above way to create $arr can also be written in PHP as:
```
$arr = array( 0=> 1, 1=>2, 2=>3);
```
Like hashes we can do things like $arr = array("joe"=> 5, "mary" =>6);

To get the keys and values we can use the functions:

$keys = array_keys($arr) and $values = array_values($arr);

Arrays can also be created by an assignment:

$barr[1] = 5; // creates array $barr if doesn't exist

If we did the assignment $barr[] = 6; then since the argument to [] wasn't specified PHP will assign $barr[2] = 6;

Yet More on PHP Arrays

You can call the unset function on the element in an array: $list= array(2,4,6,8); unset($list[2]);
Some useful array functions: count -- returns the number of elements in an array, is_array, in_array, implode, explode, sort.

To see how implode/explode work consider:

$str="this is a string";
$words = explode(" ", $str); /*acts like split except here the first argument 
  is a string rather than a regular expression. 
  So $words is an array("this", "is", "a", "string"). 
  PHP has a split function but not as fast, since 
  arg might be a regular expression. */
$str2 = implode(" ", $words); //undoes the explode.

Iterating Through Arrays

The function current can be used to return a pointer to the current element in an array. The next function can be used to advance this pointer and get its value:
```
$cities = array("San Jose", "San Diego");
echo current($cities); // prints San Jose
$another = next($cities); // $another is now San Diego;
```
There are also the functions each, prev, end, and reset to facilitate moving through array.
The function each is similar to next except after advancing the current pointer, it returns the old pointer as a two element array consisting of a key/value pair.
We saw last day that one can iterate through arrays using foreach($arr as $val){...}

PHP also supports code like

$lows = array("Mon" => 23, "Tue" => 18);
foreach($lows as $day =>$temp )
{echo "$day lows were $temp";}

Functions

The general format of a PHP functions is:
```
function name([parameter]){...}
```
For example,
```
function inc($i){return ++$i;} 
```
A return value can be sent back using a return call as in many programming languages.
You can modularize your code by putting several function definitions into a file and then use the include function to include them into any document that needs those functions.
Parameters are passed by value. So the function call:
```
$b = inc($a); // leaves the value of $a unchanged
```

You can call by reference by using an ampersand:

$b =inc(&$a); //here the value of $a is changed (one is added to it).

You can also create functions with pass by reference parameters: function inc(&$i){...}

Variable Scope

The default scope of a variable in PHP is only within the function that it is used. That is local scope: $bob = 5;
```
function test()
{ $bob = 6; echo $bob; //echo's 6}
test();
echo $bob; //echo's 5
```
In order to access global variables within a local function one would need to use the global declaration:
```
$bob = 5;
function test()
{
   global $bob; # if did not do bob would be NULL 

   echo $bob; 
}
test();
```
PHP also supports static local variables. These preserve states between function calls:
```
function addone () {static $count =0; echo $count++;}
```

Pattern Matching

PHP supports Perl style regular expression and POSIX regular expressions. For example,

 
$fruits = preg_split("/:/", "apples:oranges");
//would act like Perl's split
//preg_match acts like acts like Javascript match.

Types of web forms

Recall a basic web form looks like:

<form method="get" action="script.php">
   <input type="text" name="my_textfield" 
       size="10" />
   <input type="hidden" name="secret_data"
      value="do not peak" />
   <input type="submit" name="sendform"
      value="Send this Form" />
</form>

The method can be get or post.

The get method sends the fields as urlencoded name=value pairs appended to the URL:

script.php?my_textfield=hello&secret_data=do%20not%20peak&sendform=Send%20this%20Form

Post variables are sent as part of the content of an HTTP POST command (you won't see this in URL bar)

File upload forms must use post and in addition must set and encoding type:

<form method="post" action="script.php" enctype="multipart/form-data">
<input type="file" name="my_file" />...</form>

Built-in Globals

PHP makes available several important global variables which are useful for server side scripts.
The phpinfo() function can be used to find out all of these globals.
Here are some of the main ones:

$_SERVER
an array of information about the server like $_SERVER["SERVER_NAME"], $_SERVER ["DOCUMENT_ROOT"], $_SERVER ["QUERY_STRING"], etc

$_ENV
an array of info about the runtime environment: $_ENV["PATH"], $_ENV["PWD"], etc.

$_REQUEST
an array of the variables that have been get'd or post's from forms. So if $_SERVER["QUERY_STRING"] was hi=there&hi2=there2 would have $_REQUEST["hi"] == "there" and $_REQUEST["hi2"]="there2";

$_GET
like $_REQUEST but only for get'd variables

$_POST
like $_REQUEST but for post'd variables
PHP can be configured with register_globals = On, in which case the variable $hi would be a global with value "there". This is a bit risky security-wise.

File Reading

Since PHP is a server side technology it is allowed to create, read, and write file 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.

PHP: Functions, Patterns, Forms, Files

Outline