Introduction in php part 2

Introduction in PHP Part 2 by Bozhidar Boshnakov

Recap • We talked about – How to define variables in PHP – with $ – How to define Constants – with define(‘name’,value) – How to deal with Strings – Some predefined constants and superglobals

Table of contents • How to install a Web Server that runs PHP • How to create PHP files and run them on the browser and inside the console • Loops • Conditional statements • Functions and return values • Include and require • Variables scope

• PHP supports the C style while loop – The body of the cycle will be executed until the condition is met – The body consists of one or more statements • If more than one, surrounding brackets are required – The condition expression is of type boolean $a = 1; while ($a < 100) { $a ++; echo $a; }

• The do-while structure is similar to while-do – The condition is checked after the body is executed! – The body is executed at least once! $a = 1; do { $a ++; echo $a; } while ($a < 100); // this will produce 2 3 4 … 100 // the while cycle would output 2 3 4 … 99

• PHP supports C style for cycles – The for cycle requires initialization, iteration and ending condition statement • None of them are obligatory • Each statement can consist of multiple comma separated statements for ($i = 0; $i < 10; $i++) echo $i; for ($i = 0, $j = 10; ; $i++, $j--) if ($j > $i) echo $i; else break;

• Foreach is used to iterate over arrays – For each element in the array the body of the cycle will be called – $value will be assigned the value of the current element in the array $arr = array (1,1,2,3,5,8); foreach ($arr as $value) echo $value;

• Foreach has second form – Allows you to access the key, corresponding to the value in the array $arr = array ("one" => 1, "two" => 2); foreach ($arr as $key => $value) echo $key." => ".$value;

• You can leave a cycle with the break command • You can move immediately to next cycle iteration with continue command $i = 0; while (true) { $i ++; if ($i == 10) break; // exit the cycle if ($i%2 == 0) continue; // next iteration echo $i; } // will print out 1 3 5 7 9

• if construct allows code to be executed only if certain condition is met – Note: assignment returns as value the one being assigned. So we can have $a = 5; $b = 7; if ($a > $b) echo "A is greater than B"; if ($a % 2) { echo "A is odd"; $b = $a % 2; echo "A%2 is :".$b; } if ($b = $a%2) echo "A is odd - A%2 is :".$b;

• if-else construct is extension of if construct and allows you to execute one code if condition is met or another if not $a = 5; $b = 7; if ($a > $b) echo "A is greater than B"; else echo "B is greater or equal to A";

• Extension of the if-else construct – Allows you to add conditions for the else body – It is similar to writing else if and have two conditional statements – You can have multiple elseif statements if ($a > $b) echo "A is greater than B"; elseif ($a == $b) echo "A is equal to B"; else echo "B is greater than A";

• switch structure allows you to execute different code, depending on the value of variable – It is similar to writing a lot if-s – The switch body contains "case" clauses • The engine finds the clause that matches the value and jumps to that part of the code switch ($a) { case 0: echo "A is 0"; break; case 1: echo "A is 1"; break; }

• Similar to else, you can have default case in a switch – If no case option is found the engine jumps to the default option – The default case is not obligatory the last one switch ($a) { case 0: echo "A is 0"; break; case 1: echo "A is 1"; break; default: echo "A is … something else"; break; }

• When the engine moves to the found case it does NOT exit after the code of that case but moves on to the next one – This example will output "A is 0 A is 1" – The solution is to add break where necessary – This applies to the default case too $a = 0; switch ($a) { case 0: echo "A is 0"; case 1: echo "A is 1"; }

• Due to the behavior of the switch engine, you can use empty cases – They are without break so the engine will jump to them and move on – You can use this to combine multiple values with single code $a = 0; switch ($a) { case 0: echo "A is 0"; break; case 1: case 2: echo "A is 1 or 2"; break; }

• You can use any scalar type of variable (string, number, boolean, etc) switch ($name) { case "Dimitar": echo 1; break; case "Bozhidar": case "Boshnakov" : echo 2; break; case false : echo "No name"; break; default : echo "?!"; break; }

• Keep in mind switch uses the loose comparison "==" and may lead to unexpected results! • The solution: $v = ""; switch (true) { case ($v === false): echo "it's boolean false"; break; case ($v === 0): echo "it's numeric zero"; break; case ($v === null): echo "it's null variable"; break; case ($v === ""): echo "it's empty string"; break; }

• The ternary operator is short version of if- else construct – It is used only to return one value or another, depending on condition – The syntax is: – You cannot use it like this: <condition>?<value if true>:<value if false> echo ($a<$b ? "a is smaller" : "b is smaller"); echo ($a>$b ? "a" : "b")." is greater"; $b = ($a % 2 ? 17 : 18); ($a > 17 ? echo "a" : echo "b" );

• Functions are sets of statements, combined under unique name – Declare with statement function – Can accept parameters and return value – Helps organize and reuse the code – Echo, print and others are inbuilt functions function sum ($a, $b) { return $a + $b; } echo sum(5,7); // will output 12

• The name of the function must be unique • Can accept unlimited number of arguments – The are defined in brackets after the function name • Can return value with return statement – Accepts one parameter – the return value

• Function can have predefined value for it's parameters – Simplifies it's usage – The default value must be constant expression – The defaulted arguments must be on the right side in the function declaration! function max ($a, $b, $strict = true) { if (strict) return ($a > $b); else return ($a >= $b); } echo max(3,3,false); echo max(4,3,true); echo max(3,3); // we can omit 3rd parameter

• By default PHP passes arguments to functions by value – This means change of argument value in the function will not have effect after function ends – You can force it to pass argument by reference with & prefix of the argument function double (&$a) { $a *= 2; } $b = 7; double ($b); echo $b; // will return 14;

• PHP supports variable-length function parameters – You can pass any number of arguments to the function – The function can read the parameters with func_num_args() and func_get_arg() function sum(){ $res = 0; for ($i=0, $n = func_num_args(); $i < $n; $i++) $res += func_get_arg ($i); return $res; } echo sum (4,5,6);

• Functions can return values with the return statement – Accepts only one argument – the value to be returned – Exits the function – To return multiple values you can use arrays – Function is not obligatory to return value function foo ($a) { return true; // the following code will NOT be executed echo $a + 1; }

• You can use fixed-size arrays to return multiple values and the list statement – The list statement assigns multiple array items to variables • This is NOT a function like array • Works only for numerical arrays and assumes indexes start at 0 function small_numbers () { return array (0,1,2); } list ($a, $b, $c) = small_numbers();

• PHP supports variable functions – If variable name has parentheses appended to it the engine tries to find function with name whatever the function value is and executes it – This doesn't work with some inbuilt functions like echo, print, etc function foo () { echo "This is foo"; } $a = 'foo'; $a(); // this calls the foo function

• You can check if function is declared with function_exists(‘name’) – Useful to create cross-platform scripts • Functions can be declared inside other functions – They do not exist until the outer function is called • Functions can be defined conditionally – Depending on condition function can be defined or not

• include and require are statements to include and evaluate a file – Useful to split, combine and reuse the code – Both accept single parameter – file name – If file is not found include produces warning, require produces fatal error – File can be with any extension require "header.php"; echo "body comes here"; require "footer.php";

• include_once and require_once are forms of include and require – With include and require you can include one file many times and each time it is evaluated – With include_once and require_once if file is already included, nothing happens – For instance if in the file you have declared function, double including will produce error "Function with same name already exists"

• Variables outside function are not accessible in it – They have to be global or function must declare it will use them with global $a = "test"; function $foo () { echo $a; // this will not output anything } $a = "test"; function $foo () { global $a; echo $a; // this will output "test"; }

• Variables, declared in loops are accessible after loop is over – In the example you have to declare the array before the loop for ($i = 0; $i < 5; $i++) { $arr[] = $i; } print_r ($arr); // outputs 5; $arr = array(); for ($i = 0; $i < 5; $i++) { $arr[] = $i; } print_r ($arr); // works too

• As PHP code can be embedded in HTML, HTML code can be embedded in PHP code – This is similar to writing echo "Hello John!"; – Very useful for long texts <?php if ($name == "John") { ?> Hello John! <?php } ?>

• Resources – http://php-uroci.devbg.org/ – http://academy.telerik.com/ – http://www.codecademy.com/

1. Write a program that prints the numbers from 1 to 50 2. Write a program that prints the numbers from 1 to 50 that are not divisible by 5 and 7 3. Write a program that prints HTML table with N columns and N rows with the numbers 1, 2, 3, ... in its cells for a given N, defined as a constant 4. Write a program that finds the minimal element of an given indexed array

5. Write a program that calculates N! (factorial 1*2*..*N) for a defined constant N 6. Write a program that calculates N!*K!/(N-K)! for defined constants N and K 7. Write a program that prints the binary representation of a decimal number N, defined by a constant 8. Write a program that prints the decimal representation of a binary number, defined in a string

Introduction in php part 2

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