ITEC 325 2013spring ibarland

home—lects—exams—hws
D2L—breeze (snow day)

lect02a-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 that below.
• Variables, in php
  • All variable names must be preceded by a dollar sign ($).
  • You don't declare variables in php¹; 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|E_STRICT); 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.
• Numbers
  • floats (actually doubles) and integers
  • valid number literals
    12.2, 44, -1, -1.2
  • use:
    $count = 22; and $ctr = 13.3; …
• Numeric Operators (functions invoked w/o parentheses): +, -, *, / (always floating-point division, not quotient).
• PHP supports assignment operators.
  +=, -=, *=, /=, ++, --.
  E.g. $total += 44; Note that unlike the numeric operators, the left-hand-side must of course be a variable (not any ol' numeric-expression), just like in other languages²
• Precedence. Like other languages, PHP has a precedence order for operators. But to avoid problems with precedence in calculations, fully³ 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”⁴ 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 operators⁵ 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."; … }

      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. … }

• Arrays (See program output: lect02b-arrays.php.)

  1 <?php 2 echo "Some examples of using arrays, in php.\n\n"; 3 4 echo "----------------------\n"; 5 echo "Using arrays w/ numeric indices.\n\n"; 6 7 $arr = array("john","mary","zimbo"); 8 echo "index 1 contains ", $arr[1], "\n"; // "mary" 9 10 $arr[] = "betty"; // Insert at the *next* location. 11 echo "index 3 contains ", $arr[3], "\n"; // "betty" 12 13 $arr[23] = "justine"; 14 echo "index 17 contains ", $arr[17], "\n"; // NULL 15 echo "index 23 contains ", $arr[23], "\n"; // "justine" 16 17 $arr[] = "rachel"; // Wait, what counts as the *next* location now? 24, not 4 (!). 18 echo "index 4 contains ", $arr[ 4], "<br />\n"; // NULL 19 echo "index 24 contains ", $arr[24], "<br />\n"; // "rachel" 20 21 22 echo "----------------------\n"; 23 echo "Using arrays w/ string indices.\n\n"; 24 25 $crsarr = array("web"=>325, "java1"=>120); 26 27 echo "index 'web' contains ", $crsarr['web'], "\n"; // 325 28 29 echo "But watch out, when doing variable-substitution with arrays:"; 30 echo "index 'web' contains ", $crsarr['web'], "\n"; // 325 31 $fave = 'web'; 32 echo "index '$fave' contains ", $crsarr[$fave], "\n"; // 325 33 34 $crsarr["discretemath"] = 122; 35 echo 'index "discretemath" contains ', $crsarr['discretemath'], "\n"; // 122 36 37 // The array("john","mary","zimbo") is the same as: 38 $arr = array(0 => "john", 1 => "mary", 2 => "zimbo"); 39 40 41 /*****************************************/ 42 echo "(Reached here, 2013.Feb.06.)\n"; 43 44 echo "----------------------\n"; 45 echo "Looping over arrays\n\n"; 46 47 /* Looping over arrays: */ 48 49 function sumLengths( $data ) { 50 $lenSoFar = 0; // Include this, even if not strictly necessary! 51 foreach($arr AS $x) { 52 $lenSoFar += strlen($x); 53 } 54 return $lenSoFar; 55 } 56 57 58 /* Sometimes we need to know the index, as well as the value: */ 59 60 function myArrayToString( $data ) { 61 $stuffSoFar = ""; 62 foreach($data AS $key => $val) { 63 $stuffSoFar .= "$key maps to $val.\n"; 64 } 65 return "Array {\n" . $stuffSoFar . "}\n"; 66 } 67 68 require_once( "../Homeworks/hw02/utils.php" ); 69 70 test( 71 myArrayToString( 72 array( "en" => "hi", "fr" => "bonjour", "de" => "'tag" ) 73 ), 74 75 "Array { 76 en maps to hi. 77 fr maps to bonjour. 78 de maps to 'tag. 79 }" ); 80 81 echo "<pre>", myArrayToString( $crsarr ), "</pre>\n"; 82 83 84 echo "----------------------\n"; 85 echo "Printing arrays\n\n"; 86 87 print_r( $crsarr ); 88 print_r( array(20, 21, "howdy" => "bye", "aloha" => "sayonara", 2 => 22 ) ); 89 90 /* Of course, if printing the array-contents as html, the browser ignores newlines 91 (well, technically it treats all whitespace as a single space char). 92 The usual 'solution' is to print it inside a `pre` tag. 93 This is for q&d debugging, as the output is meant for programmers, not end-users. 94 */ 95 echo "<pre>\n"; 96 print_r( $crsarr ); 97 echo "<pre />\n"; 98 99 100 echo "<hr />\n"; 101 show_source('lect02b-arrays.php'); // This file. 102 103 ?>

• 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 Predefined Variables ($_SERVER array)
  php-ch02-predefined-vars.php
• Formatting Numbers

  • number_format(428.4959,2) == '428.50' number_format(428,2) == '428.00' number_format(123456789) == '123,456,789'

    number_format can use two other optional arguments to indicate symbols for decimal points and break thousands
    can also use sprintf and printf to format data as a string.
• While we're on related topic: Variable Type Coercion in PHP

home—lects—exams—hws
D2L—breeze (snow day)

©2012, Ian Barland, Radford University Last modified 2013.Feb.08 (Fri)

Please mail any suggestions (incl. typos, broken links) to ibarlandradford.edu