• Character Set:
  • ASCII
  • Variable identifiers are case-sensitive
  • Constant identifiers are case-sensitive
  • Class and function identifiers are case-insensitive
  • Reserved words are case-insensitive
• White Space:
  • Extra spaces, tabs, carriage returns, and new lines are (mostly) ignored

• First Character:
  • Must be a letter, underscore, or a character between 0x7F and 0xFF
• Subsequent Characters:
  • Can also be a digit
• A Restriction:
  • An identifier can not be the same as one of the pre-defined reserved words

• Simple Statements:
  • Must end in a ; (except for the last statement)
• Block/Compound Statements:
  • Consist of one or more simple statements between a { and a }
  • Need not end in a ;

• Line Based:
  • Any text between a # and the end of the line is a comment
  • Any text between a // and the end of the line is a comment
• Block:
  • Any text between a /* and a */ is a comment

• Integers:
  • Are machine-dependent (and usually correspond to the long type of the C compiler)
  • Hexadecimal integers are formatted as
    0xdigits
  • Binary integers are formatted as
    0bdigits
• Floating-Point Numbers:
  • Are machine-dependent (and usually correspond to the double type of the C compiler)

• Strings:
  • A sequence of Unicode characters
  • Can be concatenated using the . operator
  • String literals can be enclosed in either single quotes or double quotes
  • \ is the escape character and can be used to represent special characters (e.g., \n represents the newline character) and Unicode characters (e.g., \u00e9 is é)
• Booleans:
  • false, 0, 0.0, "", an array with 0 elements, and NULL all evaluate to false
  • Everything else evaluates to true

• Names:
  • A $ followed by an identifier
• Types:
  • Variables are untyped
• Syntax of Declarations:
  • Variables are undeclared (i.e., a variable is created the first time it is assigned a value)
• Notes:
  • A variable that has not been initialized behaves like NULL
  • Variables can contain names of other variables and the value of the other variable can be referenced using $$ (e.g., $name = "price"; $$name = 5; will assign the value 5 to the variable named price)

• Assignment:
  • Placing a value into the memory identified by a variable/constant
• Syntax:
  • variable = literal|expression ;
• Examples:
  • $height = 14.0; $name = "Bullwinkle";

• Assignment of a Reference:
  • Place a reference to a variable in another variable
• Syntax:
  • alias =& variable ;
• Example:
  • $height = 14.0; $tallest =& $height;

• Basics:
  • Addition (+)
  • Subtraction (-)
  • Multiplication (*)
  • Division (/)
• Integer Operators:
  • Integer Division (/)
  • Modulo (%)
• Others:
  • Increment (++) and Decrement (--)
  • Negation (-)

• Comparison:
  • Greater than: >
  • Less than: <
  • Greater than or equal to: >=
  • Less than or equal to: <=
• Sameness:
  • Equal to: ==
  • Identical to: ===
  • Not equal to: !=
  • Not Identical to: !==

• Identical for Numbers:
  • Does not perform implicit type casting
• Comparison of Strings:
  • Strings consisting of only numeric characters are compared numerically (not lexicographically)

• Basics:
  • Logical AND: &&, and
  • Logical OR: ||, or
  • Logical NOT: !
  • Logical XOR: xor
• Short Circuiting:
  • Is used

• Syntax:
  • if (boolean_expression) statement [else statement]
• Examples:
  • if ($score < 60) $grade = "F"; if ($bmi >= 30.0) $status = "Obese"; else if ($bmi >= 25.0) $status = "Overweight"; if ($quantity > 100.00) { $discount = 0.10; $note = "Volume Dicount"; }

• Syntax:
  • while (boolean) statement
• Examples:
  • $keepGoing = true; while ($keepGoing) { $count += 1; if ($count > 10) $keepGoing = false; } while ($width <= 10) { $area = $height * $width; $width += 1; }

• Syntax:
  • do statement while (boolean);
• Example:
  • do { $q = $age * $size; $age += 1; } while ($age < 65);

• Syntax:
  • for (initialization; boolean; update) statement
• Example:
  • for ($i=0; $i<size; $i++) { $area = $i * $i; }

• Defined:
  • A block of code that is defined once but invoked/executed one or more times
• Information Exchange:
  • A function may have one or more formal parameters that can be used to provide it with information
  • A function may have a return value that it evaluates to when invoked

• Syntax:
  • function [&] identifier([parameters]){ [statement;]...}
• Example:
  • function area(width, height) { $result = 0; if (($width > 0) && ($height > 0)) $result = $width * $height; return $result; }

• Syntax:
  • function([formalparameters]){ [statement;]...}
• An Example (that assigns an anonymous function to a variable named f):
  • $f = function($width, $height) { $result = 0; if (($width > 0) && ($height > 0)) $result = $width * $height; return $result; };
• Note:
  • It is not necessary to assign an anonymous function to a variable. For example, it is also possible to pass an anonymous function as an actual parameter of another function.

• Syntax:
  • identifier([actualparameters])
• Example:
  • $size = area(10.2, 20.9);
• Missing Parameters/Variable Parameter Lists:
  • One can invoke a function with any number of actual parameters (i.e., with fewer actual parameters or more actual parameters than formal parameters)
  • Hence, when writing functions, one should account for this possibility using isset(); default values (i.e., including an assignment) in the formal parameter list; and/or func_num_args() and func_get_arg()

• Global Scope:
  • A variable declared outside of a function can be accessed in any part of the program that is not in a function
  • To access a global variable inside of a function it must be declared global in that function
• Local Scope:
  • A variable declared in a function can only be accessed in that function
  • Formal parameters can only be accessed in that function
• Resolving Ambiguities:
  • Local variables take precedence over global variables
• Super-Global Scope:
  • Some variables provided by PHP have global scope and are accessible inside of functions

• Explained:
  • A static local variable retains its value between calls to the function in which it is declared
• Syntax:
  • Include the static modifier in the initialization

• Explained:
  • An identifier for a scalar value that cannot change after it has been set
• Setting the Value:
  • Use the define() function
• Example:
  • define("DEPT", "CS"); $course = DEPT . "531";
• Notes:
  • Constant identifiers are case-sensitive

• Defined:
  • An indexed array is an ordered (variable-sized) collection of (not necessarily homogeneous) values
• Terminology:
  • The numerical position is called the index (and is 0-based)
  • The value is called the element
• Literals/Initializers:
  • array([value,]...);
• Examples:
  • $grades = array(90, 85, 73, 88); $bad_idea = array("CS", 139, 4, "B-");

• Accessing Elements with the [] Operator:
  • identfier [ index ]
• Adding Elements:
  • To add an element to an array one need only assign a value to it. For example:
  • $a = array(2, 4, 6); $a[9] = 20;

• Defined:
  • An associative array is a map between a set of keys and a set of corresponding values
• Literals/Initializers:
  • array([key=>value,]...
• Examples:
  • $inventions = array("Edison"=>"Light Bulb", "Whitney"=>"Cotton Gin");

• Some Observations:
  • Arrays can be sparse
  • Using an inappropriate index/key will generate a runtime warning
• The count() and sizeof() Functions:
  • Return the number of values that are actually set

• Syntax to Access only the Value:
  • foreach (array as element) statement
• Example:
  • foreach ($purchases as $price) $total += $price;
• Syntax to Access the Key and Value:
  • foreach (array as key => value) statement
• Example:
  • foreach ($employees as $name => $salary) echo "$name earns $salary";

• An Observation:
  • Any element of an array can, itself, be an array
• Accessing Elements:
  • Use the [] operator more than once

To pass a parameter by reference, include an & before the variable name in the formal parameter list.

To pass a parameter by value, omit the &.

To return by reference, include an & before the variable name in the formal parameter list and use the =& operator to assign the result.

To return by value, omit the & and use the = operator to assign the result.

• Defined:
  • A collection of attributes/properties and methods/functions
• Constructors/Destructors:
  • __construct([parameter]..)
  • __destruct()
  • Note: There are two underscore characters!
• this:
  • A reference to the calling object
• Membership Operator:
  • ->

• Syntax:
  • class identifier { [attribute]... [method]... }
  • where an attribute consists of a visibility (public, protected, or private) and a variable initialization and method consists of a visibility and a function declaration
• Example:
  • class Course { private $dept = ""; private $number = ""; function __construct($d, $n) { $this->dept = $d; $this->number = $n; } function toString() { return $this->dept . $this->number; } }

• The new Operator:
  • name = new ClassName([parameters])
• Example:
  • $bestever = new Course("CS", "531"); echo $bestever->toString();

• Static Members:
  • Belong to the class rather than individual instances
  • Declared using the static modifier
  • Accessed externally using the class name and :: operator and internally using self and the :: operator
• Constant Attributes:
  • Declared using the const modifier (not the define() function)
  • Accessed externally using the class name and :: operator and internally using the identifier

• Syntax:
  • Include the class name (or the reserved word array) before the formal parameter
• Implication:
  • A runtime error is generated if the parameter is not of the given type (or NULL)
• Example:
  • class Student { private $courses = array(); function addCourse(Course $course) { $this->courses[count($this->courses)] = $course; } }

• Definition:
  • If class A is a subset of class B (i.e., if A has additional attributes and/or methods) then A is said to be a specialization of B
• Syntax:
  • class derivedclass extends baseclass

• Overriding:
  • If the derived class has an attribute/method with the same identifier as an attribute/method in the base class then the method/attribute in the derived calss overrides the method/attribute in the base class
• Resolving Polymorphism:
  • Normally, the interpreter searches from most specialized to least specialized
• Changing the Search Order:
  • To call an overriden method in the base class from code in the derived class use parent and the :: operator
  • To ensure that an overriden (or potentially overriden) method in the base class is called by code in the base class use self and the :: operator (rather than $this and the -> operator)

• Definition:
  • An abstract class is a class that can't have any instances (i.e., one can't, even conceptually, create instances of an abstract class)
• Abstract Methods:
  • Methods that include only a signature (i.e., no body)
• Syntax:
  • Include the abstract modifier in the declaration of the class or method

• Purpose:
  • Provide a view of a set of services provided by a class
• Interpretations:
  • An interface is a "contract"
  • A class that realizes an interface promises that it will implement all of the methods in the interface
• Syntax:
  • An interface is declared/defined just like a class but with class replaced by interface
  • A class that realizes an interface indicates that it does so using implements

• Purpose:
  • Provide a mechanism for sharing code outside of a class hierarchy
• Syntax:
  • A trait is declared/defined just like a class but with class replaced by trait
  • A class that makes use of one or more traits indicates that it does so using a use [traitname,]...; statement

• The include and require Statements:
  • Load and evaluate a specified file
  • include generates a warning and require generates a fatal error if the file is not found
• The include_once and require_once Statements:
  • Will not load/evaluate the file more than once

• Purpose:
  • Avoid name collisions
• Declaring:
  • Use the namespace statement
  • Use backslashes to create a hierarchy
• Using Namespaces:
  • Use the fully-qualified name to disambiguate if necessary
  • Creating Aliases for Fully-Qualified Names: Use the use statement

• Existence:
  • file_exists()
• Opening/Closing:
  • fopen() and fclose()
• Reading/Writing:
  • fread() and fwrite()
  • fscanf() and fprintf()