1. Lecture 5: Functions and Parameters
General syntax:
function name ( params ) {
// statements
// optional return statement }
Parameters are optional
Note that there is no return type in the header
If no return statement is used, the function will return NULL
If return is done, type could be anything
Cannot overload function names
Dynamic typing does not allow for disambiguation

2. Lecture 5: Functions and Parameters
By default, parameters are passed by value
Formal parameter is a copy of the actual parameter
Changes to formal parameter do not affect the actual parameter
If we add an ampersand before the formal parameter, we change it to pass by reference
Like in C++, but not an option in Java
Formal parameter is another name for the actual parameter
Changes to the formal parameter also change the actual parameter

3. Lecture 5: Value vs. Reference Parameters
Let's discuss this a bit
Java has only value parameters
This means we cannot alter the value stored in the actual parameter within the function
However, in many cases the value passed into the function is a reference to an object
This allows us to change the object that the reference refers to, but we cannot reassign the reference to a new object
reference cannot change this
can change this
(mutate)
data in object X

4. Lecture 5: Value vs. Reference Parameters
Reference parameters allow the data in the object to be changed
Assuming the parameter is a reference to an object
They also allow the reference to be reassigned so that the actual parameter can reference a different object
new object
reference can also be reassigned
can change this
(mutate)
old object X

5. Lecture 5: Value vs. Reference Parameters
How about object value parameters?
Can’t reassign original reference
Can change original object (only ref is copied)
Same as Java
How about array reference parameters?
Can reassign original array variable
Can change original array
Non-existent in Java
How about array value parameters?
Can’t reassign original array variable
Can’t change original array (array itself is copied)
Different from Java

6. Lecture 5: Value vs. Reference Parameters
Implications:
In Java we can alter objects, including the contents of arrays, via reference parameters
However, the parameter itself (either a primitive value or a reference) cannot change
If we want to reassign a variable within a method what can we do?
Use instance variables
Pass them in via an array (i.e. hack)
In PHP we CAN modify the value of a parameter inside a function if we use reference parameters
Be careful if you choose to do this
Avoid inadvertent changes to arguments

7. Lecture 5: Value vs. Reference Parameters
Other “by references” in PHP:
Pass by reference in foreach loop
foreach ($arrayvar as &$value)
Now you can update the contents of the array by modifying $value
More efficient than indexing into array
Assign by reference
$ref = &$var;
Now $ref is another name, or an alias, for $var
Both are bound to the same value
Unsetting a reference
unset($ref);
Now the binding between $ref and $var is broken

8. Lecture 5: Functions and Parameters
Variables within functions by default are local to the function
Like method variables in Java or automatic variables in C++
The global declaration is used to state that a variable used in a function refers to an (existing) global variable rather than a (new) local variable
The static declaration is used to state that the (new) variable has global lifetime but local scope
A “global” that prevents polluting the namespace
See ex10.php

9. Lecture 5: More on Functions
Variable Functions: PHP Function names can be stored in and called through variables
This allows the function that is used to be determined dynamically (at run time)
Can be handy if different functions need to be called in different situations
PHP functions can have default arguments
If no actual parameter is supplied the default value is used
See ex11.php

10. Lecture 5: Debugging Note
Many situations that produce compilation or run-time errors in Java will not do so in PHP
Ex: Accessing a variable that has no value:
$count = $count + 1;
Ex: Accessing a scalar as an array:
$count = 42; $count[1] = ‘hello’;
However, these situations will produce warnings, which we can elect to see or not see in the returned web page
We can determine whether these warnings (and actual errors) are seen or not via .htaccess files

11. Lecture 5: Debugging Note
These are configuration files that allow per directory configuration options for the server
For example the settings:
php_value display_errors 1
php_value display_startup_errors 1
will send PHP warnings back to the client browser
And the settings:
php_value display_errors 0
php_value display_startup_errors 0
will hide the warnings from the user
Can also set per script options for the server
ini_set('display_errors', '1');
ini_set('display_startup_errors', '1');

12. Lecture 5: CGI and Scripts
CGI - Common Gateway Interface
Interface for Web servers that interact with browsers, utilizing scripting languages and the HTTP (HyperText Transfer Protocol)
Used to allow data interaction between user and server scripts
Ex. Extracting data sent via HTTP requests and passing to scripts

13. Lecture 5: CGI and Scripts
Two best known HTTP methods: GET and POST
GET
appends user input to URL and requests corresponding document
server parses URL - first part is a program that it invokes, second part is parameters passed along
Ex.
Recommended usage for safe and idempotent requests
Safe:
For retrieval only – has no side effects on the server
Idempotent:
Making N > 1 identical requests has the same effect as making only 1 request
See:

14. POST Lecture 5: GET and POST sends data as a stream to script program
more suitable for large amounts of data
arguments are not shown in address but are still extracted and processed by server
Used for requests that may alter / update the server
i.e. NOT safe and NOT idempotent
Ex: update a database
Ex: submit a payment

15. Lecture 5: CGI and Scripts
GET and POST are often used within HTML forms
User enters data into form and then SUBMITS it
Browser processes form and passes choices and information to the url specified
Server invokes appropriate script utilizing requested method, extracting submitted data
Most scripting languages (including PHP) have predefined ways to easily extract this data
This data is used as input to the script

16. Lecture 5: CGI and Scripts
Results are sent back to browser and displayed in the Web browser
See getpost.html and getpost.php