PHP Introduction

PHP Introduction Chapter 1. Syntax and language constructs

PHP’s Type Strength • PHP is a dynamically typed language (a.k.a. weakly typed language): • The type of a variable is not / cannot be (!) declared when you create a new variable • The type of a variable is automatically determined by the value assigned to it • The type of a variable can change throughout the program according to what is stored in it at any given time $totalqty = 0; // integer $totalamount = 0.00; // float $totalamount = ‘Hello’; // string

PHP’s Type Casting • You can force a data type by type casting • Syntax: put target type in parentheses in front of variable/expression you want to cast (similar to Java): $totalqty = 0; $totalamount = (float) $totalqty; //$totalamount variable will be of type float and // cast variable doesn’t change types! • Built-in functions are also available to test and set type (see later)

Constants • Use the define() function to create a constant define("CONSTANT_NAME", value); • Constant names: • Do not begin with a dollar sign(!) • It is common practice to use all uppercase letters for constant names • Are case sensitive by default (this can be changed, but not recommended!) • The value you pass to the define() function can only be a scalar value: a string, integer, float, or boolean value define(“VOTING_AGE”, 18); echo “<p>The legal voting age is ”, VOTING_AGE,"</p>"; • Note no $ sign precedes constant names in expressions! • A constant’s name cannot be used as variable names within the quotation signs surrounding a string! • Alist of predefined vars and constants can be obtained calling phpinfo()

Variables’ Scope • PHP superglobals:

Variables’ Scope - 6 basic scope rules • Built-in superglobalor autoglobalvariables are visible everywhere (=globally) within a script, both inside and outside functions. • PHP includes various autoglobals or superglobals, which are predefined global arrays. • Autoglobals contain client, server, and environment information that you can use in your scripts. • Autoglobals are associative arrays – arrays whose elements are referred to with an alphanumeric key instead of an index number.

PHP Operators • Arithmetic operators: • Binary operators +, -, *, /, % • Usually applied to integers or doubles • String operands are converted to numbers: starting at the beginning of the string up to 1st non-valid character; if no valid characters, the value of the string will be 0. • Unary operators +, - • String operators: • . – the string concatenation operator $s1 = “Bob’s ”; $s2 = “Auto Parts”; $result = $s1 . $s2; // result now contains the string “Bob’s Auto Parts”

Operators • http://cscdb.nku.edu/csc301/frank/PHP_Crash_Course_Examples/operators.php • http://www.nku.edu/~frank/csc301/Examples/PHP_Crash_Course/operators_php.pdf

PHP Operators • Assignment operators: • Basic assignment operator = • The value of an assignment expression lhs_operand = rhs_expression is the value that is assigned to lhs_operand • =associates from right to left expressions like the following one are correct: $a = $b = 6 + $c = 9; // $cis now 9, $a and $b are 15 // canalso be written as $a = $b = 6 + ($c = 9); • Combined assignment operators: +=, -=, *=, /=, %=, .= • $a += 5; is equivalent to $a = $a +5;

PHP Operators • Assignment operators: • Pre- and Post-Increment and Decrement: ++, -- • All 4 operators modify the operand and the value of the expression is the old value (post) or the new value (pre) of the operand (like in Java). • Reference operator: & • $a = 5; $b = $a; a copy of the value in $a is made and stored in $b, elsewhere in memory • $b = &$a; $b is an alias for the same memory location which is referred to as $a = $a and $b are associated with / name the same memory location • $a = 7; // both $a and $b are now 7 • unset($b) breaks the link between name $b and the memory location storing 7 → $b is now undefined

PHP Operators • Comparison operators: • == equals === identical (if operands are equal and of the same type) != not equal <> not equal !== not identical < > <= >= • string comp_op integer → string is converted to a number 0==‘0’ →true 0 === ‘0’ →false • string comp_op string → compared as numbers if they are numerical strings…

PHP Operators • Logical operators (combine the results of logical conditions): • ! NOT && AND || OR and AND (but lower precedence than &&) or OR (but lower precedence than ||) xor EXCLUSIVE OR ($a xor $b is true when exactly one of $a and $b is true) • The execution operator • `` (= a pair of backticks, not quotes!) • PHP tries to execute the content between `` as a command at the server’s command line. The value of the expression is the output of that command. • Example: $out = `dir`; // on a Win server, the list of files/directories in current directory = where the script is echo “<pre> $out </pre>”;

PHP Operators - Precedence & Associativity • Precedence =order in which operations in an expression are evaluated • Associativity =order in which operators of equal precedence are evaluated $a=5; $b=4; ? The value of the following expression? $b + ++$a/2 + 3 == 10

Using Variable Functions • Testing and setting variable types: • string gettype(mixed variable) Returns one of the strings: “bool”, “int”, “double”, “string”, “array”, “object”, “resource”, “NULL”, or “unknown type”. “Mixed” – is a pseudo-type / signifies that variables of many (or any) data types are accepted as arguments; or function is “oveloaded” • bool settype(mixed variable, string type) Sets the type of “variable” to the new type passed as a second argument. “Type” can be “bool”, “int”, “double”, “string”, “array”, “object”, “resource”, “NULL”. $a = 56; echo gettype($a).’<br/>’; // displays int settype($a, ‘double’); echo gettype($a).’<br/>’; // displays double

Using Variable Functions • Specific type-testing functions, return true of false • is_array(variable) • is_double(variable) is_float(variable) is_real(variable) • is_long(variable) is_int(variable) is_integer(variable) • is_string(variable) • is_bool(variable) • is_object(variable) • is_resource(variable) • is_null(variable) • is_scalar(variable) – checks whether variable is an integer, boolean, string or float • is_numeric(variable) – checks whether variable is any kind of number or a numeric string • is_callable(variable) – checks whether variable is the name of a valid function

Using Variable Functions • Testing variable status: • bool isset(mixed variable1 [, mixed variable2 …] ) Returns true if all variables passed to the function exist and false otherwise • void unset(mixed variable1 [, mixed variable2, …]) Gets rid of the variable(s) it is passed • bool empty(mixed variable) Checks if variable exists and has a nonempty, nonzero value • These functions are very useful for checking (server-side) if the user filled out the appropriate fields in the form; example: echo isset($tireqty); // always true for a textbox field echo empty($tireqty); // depends on user input, false if form field is empty

PHP Conditionals • if (condition) statement or code_block where code_block = { statement(s) } • if (condition) statement or code_block else statement or code_block • NOTE: when writing a cascading set of if-else statements “else if” can be also written “elseif” • Conditional operator: condition ? value_if_true : value_if_false • switch • Same syntax as Java; control expression and case labels can evaluate to integer, string or float • Must use break to end execution after a matching case

PHP Conditionals - Example • HTML form contains the following select list: <select name="find"> <option value="a">I'm a regular customer</option> <option value="b">TV advertising</option> … </select> • Equivalent if-else and switch statements for processing form data for the select form field: • $find = $_POST["find"]; • if ($find == "a") echo "<p>Regular customer.</p>"; elseif ($find == "b") echo "<p>Customer referred by TV advertising.</p>"; … • switch($find) { case "a": echo "<p>Regular customer.</p>"; break; … }

PHP Conditionals - Example • http://cscdb.nku.edu/csc301/frank/PHP_Crash_Course_Examples/orderform_3.html • http://www.nku.edu/~frank/csc301/Examples/PHP_Crash_Course/processorder_3_php.pdf

PHP Loop Statements • while ( condition ) // same semantics as in Java statement or code_block • Example - generate a shipping cost table, if prices are proportional to the distance: <table border="0" cellpadding="3"> <tr> <td class="header" align="center">Distance</td> <td class="header" align="center">Cost</td> </tr> <?php $distance = 50; while ($distance <= 250 ) { echo "<tr>\n <td align = 'right'>$distance</td>\n"; echo "<td align = 'right'>". $distance/10 ."</td>\n</tr>\n"; $distance += 50; } ?> </table>

PHP Loop Statements • http://cscdb.nku.edu/csc301/frank/PHP_Crash_Course_Examples/freight.php • http://www.nku.edu/~frank/csc301/Examples/PHP_Crash_Course/freight_php.pdf

PHP Loop Statements • do statement or code_block while ( condition );// same semantics as in Java • for (initial-action; loop-continuation-condition; action-after-each-iteration) statement or code_block // same semantics as in Java • Interesting example – combine variable variables with a for loop to iterate through and process a series of form fields with repetitive names, such as name1, name2, name3 etc: for ($i=1; $i<=$numfields; $i++) { $temp = “name$i”; // $i is interpolated! echo $$temp . “<br />”; }

Alternative Control Structure Syntax • control_structure (expression_or_condition) : statements; special_keyword; • The alternative syntax can be used for: statement special_keyword is: • if endif • switch endswitch • while endwhile • for endfor • if ($totalqty == 0) : echo "<p>You didn’t order anything on the previous page!</p>"; exit; endif;

PHP Introduction