Introduction to PHP.

Syntactic Characteristics.

PHP has:

• four scalar types:
  
  
  1. Boolean
  
  
  
  2. Integer
  
  
  
  3. Double
  
  
  
  4. String



• Two compound types
  
  
  1. Array
  
  
  
  2. Object



• Two special types
  
  
  1. Resource
  
  
  
  2. NULL

1. Variables
   
   
   • The type of a variable is set every time it is assigned a value
   
   
   
   • An unassigned variable (unbound variable) has the value of NULL
   
   
   
   • Unbound variables appearing on an expression have their value coerced
     
     
     • To 0 if the context specifies an integer
     
     
     
     • To the empty string if the context specifies a string
   
   
   
   • The function IsSet takes a variable's name as its parameter and returns a Boolean
     
     
     • TRUE if the variable has a non-NULL value
     
     
     
     • FALSE otherwise
   
   
   
   • The function unSet sets a variable back to the unassigned state
   
   
   

   • error_reporting(15);
     

     Changes the error reporting level of the PHP interpreter
     
     so the user can be informed when an unbound variable is referenced
     
     The default level is 7



2. Integer Type
   
   PHP has a single integer type integer
   
   The size of a word size on the machine



3. Double Type
   
   Double literals can have a decimal point or an exponent or both



4. String Type
   
   
   • Characters are single bytes -- there is not character type
     
     Single characters are represented by strings of length one
   
   
   
   • String literals just like Perl's are delimited by single (') or double quotes (")
   
   
   
   • Double quoted literals can include embedded newline characters created by the Return key of the keyboard



5. Boolean Type
   
   
   • An integer expression used in Boolean context evaluates to
     
     
     • FALSE if the expression value is 0,
     
     
     
     • TRUE if the expression value is NOT 0
   
   
   
   • A string expression used in a Boolean context evaluates to
     
     
     • FALSE if the expression value is "0", or the empty string
     
     
     
     • TRUE otherwise
   
   
   
   • The string "0.0" evaluates to TRUE



6. Arithmetic Operators/Expressions
   
   
   • Arithmetic Operators: +, -, *, /, %, ++, and --
     
     
     • +, -, * produce a double(integer) for double (integer) operands
     
     
     
     • If the result of integer division is not an integer, a double is returned
     
     
     
     • The operands of the modulus operator % are coerced to integers if they are not.
   
   
   
   • Large number of predefined functions to operate on numeric values



7. String Operations
   
   
   • The only string operator is the catenation operator, a period (.)
   
   
   
   • The individual characters in a string can be accessed by their position on the string
     
     

     $str = "Yellow";  # $str{4} is "o"
     	

   
   
   
   • Large number of predefined functions to operate on strings



8. Scalar Type conversions
   
   
   • Implicit type conversions are coercions
     
     
     • Numeric values in a string context are coerced to a string
     
     
     
     • String values appearing in a numeric context are coerced to numeric values
       
       
       • If the string contains only digits, it is converted to an integer
       
       
       
       • If the string contains only digits and a period or an e/E, it is converted to an double
       
       
       
       • If the string does not begin period, a sign or a digit, it is converted to zero
     
     
     
     • When a double is converted to an integer the fractional part is dropped
   
   
   
   • Explicit type conversions:
     
     
     1. An expression can be cast to a different type
        
        

        $sum = 4.777;
        $res = (int)$sum;  # value of $res is now 4
                        

     
     
     
     2. Use type conversion functions: intval, doubleval, or strval
        
        

        $res = intval(4sum);  # value of $res is now 4
                        

     
     
     
     3. Use the settype functions which takes two parameters: a variable and a string specifying type name
        
        

        settype($sum, "integer");  # value of $sum is now 4
                        

   
   
   
   • The type of a variable can be determined
     
     
     • The gettype function takes on the variable's name as a parameter
       
       Returns a string with the name of the type of the variable
       
       It may return "unknown"
     
     
     
     • The testing type functions:is_integer, is_double, is_string, is_bool, etc.



9. Assignment operators just like C and Perl