PHP Programming Session 2 INFM 718N Web-Enabled Databases

Agenda Programming PHP Idea Rally (40 minutes) Programming well

Database Server-side Programming Interchange Language Client-side Programming Web Browser Client Hardware Server Hardware (PC, Unix) (MySQL) (PHP) (HTML, XML) (JavaScript) (IE, Firefox) (PC) Business rules Interaction Design Interface Design Relational normalization Structured programming Software patterns Object-oriented design Functional decomposition

Software Software represents an aspect of reality –Input and output represent the state of the world –Software describes how the two are related Programming languages specify the model –Data structures model things –Structured programming models actions –Object-oriented programming links the two A development process organizes the effort

The Big Picture ??

Language Learning Learn some words Put those words together in simple ways Examine to broaden your understanding Create to deepen your mastery Repeat until fluent

Thinking About PHP Local vs. Web-server-based display HTML as an indirect display mechanism “View Source” for debugging Procedural perspective (vs. object-oriented)

Making PHP HTML stuff <?php PHP stuff ?> HTML stuff stuff---/xxxxx.php

Variables Name starts with a $ –Case sensitive (assume everything could be!) Hold a value –Number (integer, float) –String (double quotes, \ escape character) –TRUE, FLASE –NULL Need not be declared, automatically cast

Operators in PHP Arithmetic operators + - * / Logical operators = > && || ! String operator.

Statements in PHP Sequential {…; …;…;} Semicolons are required at the end of every statement Conditional if (3==i) {…} else {…} Loop foreach ($array as $key => $value) {…} while ($row=mysql_fetch_array(…)) {…} For ($i=0; $i<10; $i++) {…} Braces are optional around a single statement

Arrays in PHP A set of key-element pairs $days = array(“Jan”->31, “Feb”=>28, …); $months = explode(“/”, “Jan/Feb/Mar/…/Dec”); $_POST Each element is accessed by the key –{$days[“Jan”]} –$months[0]; Arrays and loops work naturally together

Thinking about Arrays Naturally encodes an order among elements –$days = rksort($days); Natural data structure to use with a loop –Do the same thing to different data PHP unifies arrays and hashtables –Elements may be different types

Functions in PHP Declaration function multiply($a, $b=3){return $a*$b;} Invoking a method $b = multiply($b, 7); All variables in a function have only local scope Unless declared as global in the function

Why Modularity? Limit complexity –Extent –Interaction –Abstraction Minimize duplication

Using PHP with (X)HTML Forms ”, size=30 /> Yes No if (isset($_POST[“submitted”])) { echo “Your address is $ .”; } else { echo “Error: page reached without proper form submission!”; }

Sources of Complexity Syntax –Learn to read past the syntax to see the ideas –Copy working examples to get the same effect Interaction of data and control structures –Structured programming Modularity

Some Things to Pay Attention To Syntax How layout helps reading How variables are named How strings are used How input is obtained How output is created Structured Programming How things are nested How arrays are used Modular Programming Functional decomposition How functions are invoked How arguments work How scope is managed How errors are handled How results are passed

The “Idea Rally” A 90-second “elevator pitch” –One interesting and feasible idea –Plus why they should want you on their team! –Showing one optional powerpoint slide Illustrate your point, don’t say the same words! One-page handout (22 copies) –Your idea in a short paragraph –Your name, picture and contact information –A list of your strengths

First Things First Functionality Content Usability Security/Stability

What are Requirements? Attributes –Appearance –Concepts (represented by data) Behavior –What it does –How you control it –How you observe the results

Who Sets the Requirements? People who need the task done (customers) People that will operate the system (users) People who use the system’s outputs People who provide the system’s inputs Whoever pays for it (requirements commissioner)

The Requirements Interview Focus the discussion on the task –Look for entities that are mentioned Discuss the system’s most important effects –Displays, reports, data storage –Learn where the system’s inputs come from –People, stored data, devices, … Note any data that is mentioned –Try to understand the structure of the data Shoot for the big picture, not every detail

The Project Plan One-page contract –Between developer and requirements commissioner Goal The problem to be solved ProductWhat you plan to deliver ScopeAvailable time and personnel RolesWhat you expect each other to do

Programming Skills Hierarchy Reusing code [run the book’s programs] Understanding patterns [read the book] Applying patterns [modify programs] Coding without patterns [programming] Recognizing new patterns

Best Practices Design before you build Focus your learning Program defensively Limit complexity Debug syntax from the top down

Rapid Prototyping + Waterfall Update Requirements Choose Functionality Build Prototype Initial Requirements Write Specification Create Software Write Test Plan

Focus Your Learning Find examples that work –Tutorials, articles, examples Cut them down to focus on what you need –Easiest to learn with throwaway programs Once it works, include it in your program –If it fails, you have a working example to look at

Defensive Programming Goal of software is to create desired output Programs transform input into output –Some inputs may yield undesired output Methods should enforce input assumptions –Guards against the user and the programmer! Everything should be done inside methods

Limiting Complexity Single errors are usually easy to fix –So avoid introducing multiple errors Start with something that works –Start with an existing program if possible –If starting from scratch, start small Add one new feature –Preferably isolated in its own method

Types of Errors Syntax errors –Detected at compile time Run time exceptions –Cause system-detected failures at run time Logic errors –Cause unanticipated behavior (detected by you!) Design errors –Fail to meet the need (detected by stakeholders)

Debugging Syntax Errors Focus on the first error message –Fix one thing at a time The line number is where it was detected –It may have been caused much earlier Understand the cause of “warnings” –They may give a clue about later errors If all else fails, comment out large code regions –If it compiles, the error is in the commented part

Run Time Exceptions Occur when you try to do the impossible –Use a null variable, divide by zero, … The cause is almost never where the error is –Why is the variable null? Exceptions often indicate a logic error –Find why it happened, not just a quick fix!

Debugging Run-Time Exceptions Run the program to get a stack trace –Where was this function called from? Print variable values before the failure Reason backwards to find the cause –Why do they have these values? If necessary, print some values further back

Logic Errors Evidenced by inappropriate behavior Can’t be automatically detected –“Inappropriate” is subjective Sometimes very hard to detect –Sometimes dependent on user behavior –Sometimes (apparently) random Cause can be hard to pin down

Debugging Logic Errors First, look where the bad data was created If that fails, print variables at key locations –if (DEBUG) echo “\$foobar = $foobar”; Examine output for unexpected patterns Once found, proceed as for run time errors –define (“DEBUG”, FALSE); to clean the output

Three Big Ideas Functional decomposition –Outside-in design High-level languages –Structured programming, object-oriented design Patterns –Design patterns, standard algorithms, code reuse

One-Minute Paper What was the muddiest point in today’s class? Be brief! No names!