Lecture 5. Monday, February 1, 2016

Transcription

1 Lecture 5 Monday, February 1, 2016

2 === and!== In an example last week, we talked about how PHP does not check type when doing comparisons (so 1.0 == "1.0" is true) PHP calls this type juggling it juggles types of operands around to make them the same before performing a comparison Sometimes, we may need to check that type 1

3 === and!== For example, suppose we have the following script: <?php $a = 100 $b = "1e2"; // scientific notation for 10 2 echo $a == $b? 1 : 0;?> Output will be 1 2

4 === and!== We have the operator === (identical) $a === $b checks that $a and $b are both equal and they are of the same type Examples: o 1.0 === 1.0 => true o 1.0 === "1.0" => false o 1.0 === 2.0 => false o 1.0 === "2.0" => false 3

5 Similarly, we have!== === and!== $a!== $b will return true if $a is not equal to $b or they are not the same type Examples: o 1.0!== 1.0 => false o 1.0!== "1.0" => true o 1.0!== 2.0 => true o 1.0!== "2.0" => true 4

6 != Another operand we may see is <>, which is the same as!= This is the same operator used in Excel and some other programming languages where they do not use!= If you think about this logically, if we were to say the statement $a <> $b, we would say a is less than or greater than b If a number is less than or greater than another number, it means they cannot be equal 5

7 Debugging Variable Types To help debug variable types, we can use the function var_dump($variable) Examples: o var_dump(1) => int(1) o var_dump(1.0) => float(1) o var_dump("1") => string(1) "1" o var_dump("1.0") => string(3) "1.0" o var_dump(array(1)) => array(1) {[0] => int(1)} 6

8 Operators Coming in PHP 7 While we do not have PHP 7 yet, there are two key operators that have been added that may be helpful should you go on in PHP 7

9 PHP 7: Spaceship Operator The operator <=> has one of three return values: o $a <=> $b == -1 => $a < $b o $a <=> $b == 0 => $a == $b o $a <=> $b == 1 => $a > $b This works for both numerical values, but also strings 8

10 PHP 7: Null Coalescing The?? operator can be stacked with an infinite* number of operands, and returns the left-most operand that is not empty and not equal to null If all operands are null, it results in null 9

11 PHP 7: Null Coalescing Suppose you had a variable $x that could be submitted by a form through POST, a query string through GET, or set to a default value (10) in that order of precedence We would probably write the code: <?php if(isset($_post["x"])) $x = $_POST["x"]; else if(isset($_get["x"])) $x = $_GET["x"]; else $x = 10;?> 10

12 PHP 7: Null Coalescing With the?? operator, we would be able to rewrite this as: We would probably write the code: <?php $x = $_POST["x"]?? $_GET["x"]?? 10;?> 11

13 Error Logs Last week, if your code had any PHP errors, you received a 500 Server Error, rather than the output of the error This is not normal But, here s why it happened 12

14 Error Logs Error logs are stored in /var/log/apache2/error.log All files on a UNIX server are, of course, owned by somebody They are owned by root; however, the www-data user is configured to allow it to write to that file However, your account is not configured to be allowed to write to that file Even if we could write to them through this process, it would be problematic because we wouldn t be able to read them, either 13

15 Error Logs In the virtual host configuration, we can tell it to use a different location to store error logs specifically, one that it accessible to us Your virtualhosts will be reconfigured to put the error log in ~/vhost_errors.log WARNING: Do not delete this file, as it does not reappear until the webserver is restarted 14

16 include() and require() In Lab 2, we reused code we wrote for the recursive factorial function This (reusing code) will occur frequently We can include the code from another file inside another 15

17 include() and require() There are two functions include and require The difference: o include will give a warning message if the file is not found but continue executing the script o require will through a fatal error and execution of the script will stop 16

18 include() and require() In either case, you re likely to get a fatal error eventually, unless you are including files that aren t necessary For example, if you re including a file with functions you ve written and the file you re attempting to include doesn t exist, you ll either get an error loading the file or an error using the function For this class, you should always use require, as it makes it easier to debug our programs 17

19 include() and require() When you require a file containing PHP code, the PHP code in the included file must still be enclosed in PHP tags or it will not execute Code that is not wrapped in PHP code will be displayed in HTML 18

20 include() and require() Suppose you have the file, header.php, that just contains the following (no PHP tags): print "<h1>page Title</h1>"; In another file, you have: <?php require("header.php");?> It does not just substitute the contents of header.php in to the already-opened PHP tags It would display on the webpage: print "<h1>page Title</h1>"; 19

21 require() vs. require_once() For both require and include, there are matching functions, require_once and include_once The _once functions ensure that the file is included only one time, even if you call require_once on that file multiple times 20

22 require() vs. require_once() Example: o Suppose you have a web application that uses stored credentials to connect to both a MySQL database and an LDAP login server o Somewhere you have a file that contains all of your functions called functions.php o In files called mysql.php, you have variables for the MySQL login credentials, and it also loads your functions.php file for the functions you wrote to generate MySQL queries o In a file called ldap.php, you have variables for the LDAP login credientials, and it also loads your functions.php file for the functions you wrote to generate LDAP queries o Your file that processes submissions from the login form includes both mysql.php and ldap.php o These files will (both) attempt to load functions.php, which will give an error the second time because you ve redefined functions 21

23 require() vs. require_once() Why might you want to require/include a file multiple times? o Suppose you had a file, calculate.php, that has the following code in it: <?php $x += 8;?> o Want to perform that calculation twice? <?php $x = 10; require("calculate.php"); require("calculate.php"); print "$x\n"; // prints 26?> Although it might make more sense to just make the contents of calculate.php a function, there are some instances with objects where that isn t easy do 22

24 Strings Similar to other languages, we can think of a string as an array of characters We can access the third character in the variable $str using $str[2] We can also change it using $str[2] = 'X'; Although some languages will accept a range, like $str[2:4] to get multiple characters, PHP does not 23

25 Common String Functions trim($str) strlen($str) strtoupper($str) strtolower($str) ucfirst($str) ucwords($str) substr($str, $start, $length) str_replace($search, $replace, $str) explode($del, $str) implode($glue, $pieces) str_pad($str, $length, $pad) strpos($haystack, $needle, $offset=0) [stripos()] strcmp($str1, $str2) [strcasecmp()] 24

26 Multi-Dimensional Arrays PHP was created before SQL databases were readily available, so making arrays functional was a key component of the PHP language When you consider what we ve learned about PHP, we ve seen we can create associative arrays (where we define a number or string as the key, which we typically see as a dictionary in other languages), or the standard indexing starting at 0 (usually called arrays in other languages, but sometimes lists) If you read PHP s documentation, they will often refer to multi-dimensional arrays as tables, the first index of a multidimensional array as a row, the second index as a column, etc. 25

27 Multi-Dimensional Arrays We can define multidimensional arrays like this: <?php $a = array(array(13, 17, 19), array(23, 29, 31));?> In this example, $a[0][1] = 17, $a[1][0] = 23 26

28 Multi-Dimensional Arrays We can still define our own keys, and we don t need to necessarily have the same keys in all of the arrays at the same level: <?php $a = array( "j" => array(1 => 13, 2 => 17, 3 => 19), "k" => array("a" => 23, "b" => 29, "c" => 31) );?> 27

29 Multi-Dimensional Arrays But going back to what we said about tables, this is where the power with multi-dimensional arrays comes into play: <?php $a = array( array("sub" => "CS", "num" => "383", "title" => "Web Development II"), array("sub" => "MTH", "num" => "111", "title" => "Calculus I"), );?> But going back If we think about this in terms of a database, the $a[0]["title"] has the value Web Development II, which we can think of as the title field/column in record/row 1 28