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PHP: Learn PHP in 3 Hours

Xah Lee

, 2009-01-26, …, 2013-03-27

This page is a PHP tutorial, language in a glance. Example here are based on PHP version 5.x. For installation, see PHP Install.

Code Markers, Comment Syntax

PHP code must be enclosed between delimiters like this <?php … ?>. Anything outside of it is printed as is.

<?php
# this is comment. (bash style)

// comment too, start with 2 slashes. (C style)
5 + 4;

/* Java, C++ style,
multi-line comment is also supported. */

?>

hi there

Save the above as myfile.php. You can run it in the command line like this: php myfile.php, or, you can run it in browser thru http server. The result output is: “hi there”.

print version

<?php
echo phpversion();

// print many config info
phpinfo();
?>

Strings

Quoting String

use single quote for literal string.

<?php
# single quoted string's content is literal.
# The \n will be printed as blackslash and n
echo 'hi there\n something';
?>

use double quote for interpreted string. That is, use \n for line break, and $‹var name› for variable value.

<?php
$x = 4;

/* double quoted strings are interpreted. \n is a new line. Variable become values */
echo "I have $x apples\n"; // prints “I have 4 apples” with a newline
?>

<?php
// string can contain Unicode characters, or literal line break
echo "α β
and ♥";
?>

If you are using Unicode, your HTML should declare Unicode encoding. See: Character Sets and Encoding in HTML.

String Operations

join string

<?php
// use period to join string
echo "Once " . "upon a time";
?>

substring

<?php
// substring, use substr( $myStr, $startIndex, $length). Index starts with 0
$aa = "012345";
echo substr($aa, 0, 4); // prints “0123”
?>

string length

<?php
// string length
$aa = "once";
echo strlen($aa); // prints 4
?>

string functions ◇ String Operators

substr

Global and Local variables

Variables do not need to be declared. All variables must have a dollar sign “$” in front. (variable always have a dollar sign in front, and dollar sign only. Unlike perl, the dollar sign (aka “sigil”) does not change depending on the value of the variable. In PHP, there's no perl's concept of “context”.)

<?php
$xx = 4;
echo "value of xx is $xx";
?>

Basically all variables are global. Variables inside functions definition are local to that function.

<?php
$x = 4;

function xx()
{
  echo "value of x is $x"; # x here is null
}

xx(); # prints: "value of x is "
?>

To refer to global vars inside function definition, declare it first by putting “global” in front of the var.

<?php
$x = 4;

function xx()
{
    global $x;
    echo "→ $x";
}

xx(); # prints → 4
?>

Using a curly brace block “{…}” does NOT make variables inside it local.

language.variables.scope

True ＆ False

true and false are built-in boolean type. Case does not matter. true is the same as True.

The following are all FALSE:

the integer 0 (zero)
the float 0.0 (zero)
the empty string "", and the string "0"
array with zero elements
object with zero member variables (PHP 4 only)
the special type NULL (including unset variables)
SimpleXML objects created from empty tags

Example:

<?php
$x = 4;
if ($x == 4) {echo "yay";} else {echo "nay";} # yay
if (true) {echo "yay";} else {echo "nay";} # yay
if (trUe) {echo "yay";} else {echo "nay";} # yay
?>

If Then Else

simple if.

<?php
$x = 4;
if ($x <= 4) {echo "yay!";}
?>

if else.

<?php
$x = 4; $y = 5;
if ($x == $y) {echo "yay";} else {echo "nay";}
?>

elesif

<?php
$a=3; $b=4;
if ($a > $b) {
  echo "case 1";
} elseif ($a == $b) {
  echo "case 2";
} else {
  echo "case 3!";
}
?>

control-structures

Loop, Iterations

while loop.

<?php
$i = 1;
while ($i < 5) {
  echo "$i\n";
  $i++;
}
?>

for loop.

<?php
for ($i = 1; $i < 5; $i++) {
    echo $i;
}
?>

Keyword “break” can be used to exit a loop. Example:

<?php
for ($i = 1; $i < 8; $i++) {
  echo $i;
  if ($i == 4) {break;}
}
?>

control-structures

break

Lists (Array)

PHP combines the concepts of “array/list” and “hash/keyed-list” into one. It is just called “array”.

A PHP array is just a sequence of things. Each element is actually a pair, made of a “key” and a “value”. The “key” can be omitted if you don't need a list of pairs. If a key is omitted, they are automatically generated using incremental indexes starting from 0.

Here are examples of manipulating array as simple list without using the “key”.

Array is created using array(). Example:

<?php
$x = array(7,2,3,"yes",5);
print_r($x);

/*  prints:

Array
(
    [0] => 7
    [1] => 2
    [2] => 3
    [3] => yes
    [4] => 5
)
 */

?>

“print_r” is for printing array or other variable in a human-readable way.

print_r

Array as Pairs/Hash/Keyed-List

For using array as a keyed-list (aka hash-table, dictionary, associative list), see keyed list.

Length

Use count() to get the number of elements.

<?php
$x = array(7,"yes",5);
echo count($x);  // prints 3
?>

Getting a Element

Extracting element can be done like this $myArray[‹key›].

<?php
$x = array(7, 2, 3, "yes", 5);
echo $x[1]; // prints 2
?>

Replacing a Element

<?php
$x = array(7, "yes", 5);
$x[1] = "no";
echo $x[1];  // prints no
?>

Appending a Element

Adding a element is done like this $myArray[‹new key›] = ‹new value›. The ‹new key› must be a key that does not exist already, otherwise it simply replace that key's value.

If you are using array as simple list without keys, and have never removed any element in your list, you can add new ones with automatic index like this: $myList[] = ‹new value›.

<?php
$x = array("uni", "bi", "tri");
$x[] = "quad"; // appending a element
print_r($x);

/* 
Array
(
    [0] => uni
    [1] => bi
    [2] => tri
    [3] => quad
)
*/
?>

Removing Elements

To delete a element, use unset().

<?php
$x = array(7, "yes", 5);
unset($x[1]); // removes the "yes"
print_r($x); // element with index 1 no longer exist.

/* output

Array
(
    [0] => 7
    [2] => 5
)
*/
?>

WARNING: Once a element is deleted, there no longer exist a element with that element's index. In other words, the array is not “re-indexed”, and in fact there is no such concept as “re-indexing”. If you want the index to be sequential starting from 0 again, just create a copy like this: $newA = array_values($oldA);.

Nested List

Arrays can be nested.

<?php
$x = array(7, "yes", 5);
$y = array("woot", $x, "wee");
print_r($y);

/* output

Array
(
    [0] => woot
    [1] => Array
        (
            [0] => 7
            [1] => yes
            [2] => 5
        )

    [2] => wee
)
*/

?>

To get a element from a nested array, use the form $myArray[‹key1›][‹key2›][…]…. Example:

<?php
$x = array(7,"yes",5);
$y = array("woot", $x, "wee");
echo $y[1][0]; // prints 7
?>

Arrays ◇ Array Operators ◇ Array Functions

Keyed-List

A keyed list (aka hash-table, dictionary, associative list) in PHP is simply called array. It is constructed like this array(‹key1› => ‹val1›, ‹key2› => ‹val2›, …). Example:

<?php
$x = array("mary" => 19, "jane" => 16);
print_r($x);

/* 

Array
(
    [mary] => 19
    [jane] => 16
)
*/
?>

Get Values

To get value of a element, use the form $myArray[‹key›].

<?php
$x = array("mary" => 19, "jane" => 16);
echo($x["mary"]);
?>

Note that the key should be quoted, when it is not a number.

Modify/Add A Entry

To add a entry, use the form $myArray[‹key›] = ‹val›. If the key exist, old value will be replaced. Otherwise, a new entry will be added.

<?php
$x = array("mary" => 19, "jane" => 16);
$x["mary"] = 18; // modify a entry
$x["vicky"] = 21; // add a entry
print_r($x);
?>

Delete a Entry

To delete a entry, use unset().

<?php
$x = array("mary" => 19, "jane" => 16);
unset($x["mary"]);
print_r($x);
?>

Check Key Exists

To check if a key exists, use array_key_exists().

<?php
$x = array("mary" => 19, "jane" => 19);
echo array_key_exists("mary", $x);
?>

Get Just Keys

To get just the keys, use array_keys().

<?php
$x = array("mary" => 19, "jane" => 16);
print_r(array_keys($x));

/*
Array
(
    [0] => mary
    [1] => jane
)
*/
?>

Get Just Values

To get just the values, use array_values(). The function array_values() effectively replaces all the keys by numerical indexes starting from 0.

<?php
$x = array("mary" => 19, "jane" => 16);
print_r(array_values($x));

/* 
Array
(
    [0] => 19
    [1] => 16
)
*/
?>

Loop thru Values

To go thru a list, use foreach. Here's a example of looping thru a list and accessing its values:

<?php
$aa = array("uni", "bi", "tri");
foreach ($aa as $x) {
  echo "$x ";
} // prints uni bi tri
?>

Loop thru Keys ＆ Values

Here's a example of looping thru a list and accessing both keys and values:

<?php
$aa = array("uni", "bi", "tri");
foreach ($aa as $k => $v) {
  echo "$k,$v;  ";
} // prints 0,uni;  1,bi;  2,tri;
?>

foreach

Map thru Values

Map a function to a array.

<?php
function ff($n) {return($n+1);}

$a = array(3,9,4);
$b = array_map( ff, $a); // $b is (4,10,5)
print_r($b);
?>

array-map

Define a Function

A function is defined using keyword function. Like this:

<?php
/* ff($x,$y) returns x + y */
function ff($x,$y) {
  return($x+$y);
}

echo ff(3,4); // prints 7
?>

Optional Parameters

<?php
/* ff($x,$y) returns x + y. If $y is not given, default to 1. */
function ff($x,$y=1) {
  return($x+$y);
}

echo ff(3); // prints 4
?>

Functions

Module

A collection of functions can be written and saved into a file. This file can then be loaded using require().

For example, save the following in a file and name it myPackage.php.

<?php
function ff($x) {
  return($x+1);
}
?>

Now, eval the following code:

<?php
require("myPackage.php");
echo ff(3); // prints 4
?>

require() works as if the whole file's content is inserted in-place.

As of PHP 5.2.4 (2007-08-30), there is no namespace mechanism for importing functions or packages.

function.require

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2009-01