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lect06-php-types
ch02: php types

From PHP Visual Quickstart Guide by Larry Ullman
Originally based on notes by Jack Davis (jcdavis@radford.edu)

---

• Values, in php
  • The types of values, in php boolean, integer, float, string, array, object, resource, NULL
  • However, php converts types freely as needed (often converting from/to strings); we'll revisit this later.
• Variables, in php
  • All variable names must be preceded by a dollar sign ($).
  • You don't declare variables in php; you create a variable simply by initializing it: $myAge = 23;

  • If you use a variable that hasn't been initialized (whether intentional or not), that variable is created and has the value NULL.

    By default, using an uninitialized/undeclared variable is not an error!
    (This is unfortunate, since it means misspelling a variable-name turns into a hard-to-track bug.)

    rule-of-thumb: In all php files, include (near the start) error_reporting(E_ALL); since you want to know when you are using an undeclared variable. (Later, we'll put this command and a few others into a file startup.php, and then all our files will start with require_once('startup.php');.

  • PHP is a dynamically typed language (as is Javascript); a variable can hold any type (not restricted to one a single type, as variables in Java are).
  • Variable names are case sensitive.
  • By convention, almost all php programmers use lower case variable names, reserving uppercase names for files, resources, and constants.
• Numeric types: integers and floats (doubles):
  e.g. 44, -1, 12.2, -1.2e7
• Numeric Operators (functions invoked w/o parentheses): +, -, *, / (always floating-point division, never quotient).
• Precedence. Like other languages, PHP has a precedence order for operators. But to avoid problems with precedence in calculations, fully¹ parenthesize expressions (just like you should in other languages).
• Some numeric functions:

  • round(4.35) == 4 round(4.289,2) == 4.29 $num = 236.26985; round($num) == 236

  • ceil rounds to the next higher integer: ceil(44.3) == returns 45.
    floor rounds to the next lower integer: floor(44.9) == 44.
    Note that floor differs from truncation on negative numbers: floor(-3.5) == -4.
  • min and max will often save you an if-else.
  • The php manual lists many more math functions that can be used; we've just reviewed some of the frequently used ones.
• Strings
  • A string (or “string-literal”) is any number of characters enclosed within a pair of either single (') or double (") quotation marks. Strings can contain any combination of letters, numbers, symbols, and spaces.
    Note that unlike many languages, string-literals can contain contain newlines:

    <?php echo "hello there everybody!"; echo ' -- Dr. Nick '; ?>

  • Rule of thumb: Use double-quoted strings most of the time, in your program.
    We'll see their differences; experience suggests that double-quoted strings are more convenient for most purposes, in php.
  • If using double-quotes, the and you want to include a double-quote in your string-literal, precede that character with a backslash \". You can also embed special characters like newlines by using \n, so 'hi\nbye' is a six-character string, not seven. This is what you might be used to from most other programming languages.
  • If using single-quotes, the and you want to include a single-quote in your string-literal, precede that character with a backslash \', sensibly enough.
    However, note that no other characters can be quoted: 'hi\nbye' and hi\"bye" are both seven-character strings, not six!
  • Just to keep things odd: double-quoted literals allow most standard quoted characters except for \' so "hi\'bye" is a seven-character string, not six!
  • No matter what type of string you're using, don't try to quote the other type of quote-marks.
  • Furthermore: If a string-literals delimited with double-quotes contains variables (a dollar-sign), then the variable gets replaced with its value!
    This “interpolation”² occurs (only) within double-quoted strings, not single-quoted. This is feature of php, to make string-generation easy.
    Note: I suggest using double-quotes for most php strings, by default. For HTML, we'll tend to use single-quotes
  • String examples:
    

    $str = "How are you?"; $first_name = "John"; $str = "How are you " . $first_name . "?"; $str = "How are you $first_name?"; // alternately $date = "1/01/2010"; // this is a string! $str2 = "I said, \"how are you?\"";

  • Simple example
    php-ch02-simple.php
• Booleans: the two values true, false (case-insensitive), and standard operators !, && (boolean-and), and || (boolean-or).
  • Caution: There are also the boolean operators with spelled-out names, AND and OR. Do not use them! They are just like &&, || except that they have unexpectedly low precedence (less than the assignment operators³ Use parentheses to force precedence if you are doing something unusual (or even, just use them routinely so you never even need to think twice about precedence!).
  • A note on implicit boolean conversions:
    • In a boolean context, most values (like 17 and 'hello') are “true-ish”.
    • However, the following get interpreted as false:
      • false (duh)
      • 0
      • 0.0
      • '' (okay)
      • '0' (!!)
      • array() (an empty array) (!!)
      Some examples:

      if (!$fname) { // We reach here if $fname was undefined *or* the empty string. echo "No first name provided."; … } if (!$_POST['fname']) { // If this program is handling a web-form submitted via a 'POST' action: echo "On a web-form, the tag with name='fname' was left empty."; echo "(Presumably it was an <input> tag)."; … }

      Similarly,

      $dbConnection = mysql_connect('localhost', 'some-sql-user-name', 'some-sql-password'); if ($dbConnection) { // we reach here only if $dbConnection is a valid connection object. … }

• Named Constants
  define ("PI", 3.1415926535);
  The name of the constant is always quoted, but the value you assign to the constant is only quoted if it is a string literal. When you use a constant name, you don't precede it with a $.
  
  

  define ("PI",3.14152926535); echo "The constant PI holds ", PI, "<br/>\n";

• PHP superglobals: PHP uses the term "superglobal" in the way that most languages might use the term "global"⁴: variables that can be accessed anywhere (from any file).

  One useful superglobal is the array $_SERVER: php-ch02-predefined-vars.php. Another is an array which holds form-info provided via the http header: $_POST (and/or $_GET). The web-server initializes all these superglobals when it first fires up the php process. If you aren't running php via a web server, these superglobals will be NULL.

$n = 17

function f($x) {
  global $n; // this line required to use $n inside this function.
  return $x + $n;
  }
          

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©2014, Ian Barland, Radford University Last modified 2015.Feb.09 (Mon)

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