Intro to PHP 2

Dr. Andrew Besmer

About PHP
Type Juggling

About PHP

Variables

PHP variables are loosely or weakly typed
- It is not necessary to specify the type prior to using or initializing the variable
- The type is determined by the language based on the context of use
Variables are always assigned by value (more later)

<?php
$x = "Hello World";
$x = 0;
$x = true;

Variables

Variables do not need to be declared prior to use
This can lead to unexpected errors
Results can be predictably unexpected

<?php
$aardvark = 55;
if ($aardvrak < 50)
{
    echo "It is less than 50!";
}
elseif ($aardvark > 50) //no space!
{
    echo "It is more than 50!";
}
else
{
    echo "It must be 50!";
}
//Outputs: "It is less than 50!"
var_dump($aardvrak);
var_dump((int)$aardvrak);

Variable Primitives

Primitive Data Type: A built in data type provided by the programming language being used.¹

PHP has a total of eight primitives
- 4 scalar
- 2 compound
- 2 special
In documentation for PHP you may see some “pseudo-types”, these are used for readability of documentation
- mixed
- number
- callback

Scalars

Scalar: A variable limited to a single value at a time.

Differentiated from complex data types like array or object

Boolean

boolean: A boolean value is either true or false alternatively B = {0, 1}

The following values are considered to be false²
- false
- 0
- 0.0
- "" or "0"
- array()
- NULL
All others are considered true including -1

Integer

integer: Z = {…, -3, -2, -1, 0, 1, 2, 3, …}

Can be specified in binary, octal, decimal, or hex with a + or - indicating sign
- A 0b indicates binary
- A 0 indicates octal
- A 0x indicates hex

<?php
$bin = 0b10100011; //163 in decimal
$oct = 0123; //83 in decimal
$dec = 123; //123 in decimal
$hex = 0x64; //100 in decimal

var_dump($dec == $oct); //is bool(false)

Integer

The maximum integer size can be determined using the constant PHP_INT_SIZE
- Returns the number of bytes allocated for the variable
- Usually 8 bytes on 64 bit machines and 4 on 32 bit machines
- No “unsigned” integers in PHP
  - Signing uses the first bit to indicate positive or negative
  - What would happen if PHP did support them?
PHP_INT_MAX can be used for the maximum value

Integer

Invalid octal specification results in stopping at bad digit

<?php
var_dump(011901); //Decimal 9!

Variables do ~~not~~ need to be initialized
- Always initialize your variables even though you dont “have to”

Integer Overflow

Unlike other languages integer overflow results in using a float using E notation
Typed languages generally roll over because of 2’s complement

Float

`float` aka `double`

float: R = {x | x is a real number}

Can be specified by providing E notation or number with decimal place
Floats like integers depend on the platform, a max of ~1.8e308 with ~14 digit precision per IEEE 64bit standard

Float

Conversion to binary results in loss of precision for some numbers

This can lead to confusing results: for example, floor((0.1+0.7)*10) will usually return 7 instead of the expected 8, since the internal representation will be something like 7.9999999999999991118....³

Float

Comparing floats for inequality can also be problematic
- Can specify precision of equality⁴

<?php
$a = 1.23456789;
$b = 1.23456780;
$epsilon = 0.00001;

if(abs($a-$b) < $epsilon) {
    echo "true";
}

NaN

NaN: Not a number, can be any number of values and should not be used for any purpose other than observing that the result of the operation was undefined or unrepresentable.

Can check for NaN using is_nan()

String

string: A series of characters.

Native strings only support ASCII (no native Unicode)

<?php
$name = "Andrew Besmer"; //My name uses 13B
$name = "&rew Besmer"; //My 1337 name uses 11B

The max string length is 2GB in PHP

String

You can specify a string by using
- Single quoted syntax
- Double quoted syntax
- heredoc syntax
- nowdoc syntax
Depending on the method used variables may be inserted into the string!

String

Single quoted uses the ' character to start and end the string

<?php
$name = 'Andrew Besmer';

Variables are not inserted into the string with single quotes

<?php
$greeting = 'Hello';
$name = '$greeting Andrew Besmer';
echo $name; //Outputs: $greeting Andrew Besmer

String

What if I wanted to set the name Pat O'Neal?

\ can be used as an escape character
\' makes ' and \\ makes \

<?php
$name = 'Pat O\'Neal';
echo $name; //Outputs: Pat O'Neal

String

A double quote " can also be used to specify start and end of strings
Variables are inserted into string

<?php
$greeting = "Hello";
$name = "$greeting Andrew Besmer";
echo $name; //Outputs: Hello Andrew Besmer

String

Other common escape characters

Sequence Meaning

\r CR

\n LF

\t HT

\v VT

\\ backslash

\$ dollar sign

\" double-quote
Remember that a CRLF in HTML does nothing!

Sequence	Meaning
\r	CR
\n	LF
\t	HT
\v	VT
\\	backslash
\$	dollar sign
\"	double-quote

String

You can also use heredoc syntax <<<IDENTIFIER which will accept a string until seeing the IDENTIFIER;
Since a double quote is not used to start and stop the string definition it is not necessary to escape them
Can optionally be specified using <<<"IDENTIFIER" more explicitly explaining what will happen in the string

<?php
$greeting = "Hello!";
$longText = <<<EOF
$greeting
This is some longer text.
All of this will wind up in the string.
If can go on for many lines.
EOF;

String

Nowdoc is similar to heredoc but is specified using <<<'IDENTIFIER' instead
The ' explicitly describes expected functionality

<?php
$greeting = "Hello!";
$longText = <<<'EOF'
$greeting
This is some longer text.
All of this will wind up in the string.
If can go on for many lines.
EOF;

String

A strings character can be accessed using array syntax $name[0]
Note that arrays are 0 based
More on arrays later
Strings serve as PHP’s byte
An empty string is NULL

<?php
$emptyString = ''; //NULL

String

Warning

Strings are concatenated using the . operator NOT the + operator which many other languages use

Compound

PHP has two compound data types
- array
- object
We will learn more about both of these later

Special

PHP has two special data types
- resource
- NULL

Resource

resource: A variable to hold references to external resources, e.g. a opened files, database connections, etc…

NULL

NULL: Represents a variable with no value.

A variable is NULL if
- You explicitly assign NULL
- If you have not set any value yet
- The variable has been unset()
NULL is not case senesitive
Can check for it using is_null()

Type Juggling

PHP will auto convert the type depending on the context
PHP does not change the variable itself but it’s use in the expression and the resulting data type

<?php
$test = "100"; //A string
$test = $test + 10; //An integer
$test = $test + 10.5; //A float
$test = $test + "15 hundred"; //A float 135.5
$test = 100 + "15 hundred"; //An integer 115

Casting

It may be beneficial for you to explicity set the datatype
This can be accomplished by using casting
Cast by putting the data type you desire in parathensis
- (int), (integer)
- (bool), (boolean)
- (float), (double), (real)
- (string)
- (array)
- (object)
- (unset)
- (binary)

Casting

For boolean see earlier for how values are determined to be true or false
Examples⁵

<?php
var_dump((bool) "");        // bool(false)
var_dump((bool) 1);         // bool(true)
var_dump((bool) -2);        // bool(true)
var_dump((bool) "foo");     // bool(true)
var_dump((bool) 2.3e5);     // bool(true)
var_dump((bool) array(12)); // bool(true)
var_dump((bool) array());   // bool(false)
var_dump((bool) "false");   // bool(true)

Casting

When converting from boolean to integer
- false becomes 0
- true becomes 1
From float to integer
- The number is rounded down towards zero
- Numbers to large are undefined or NaN no errors are thrown
- Warning!
```
<?php
echo (int) ( (0.1+0.7) * 10 ); //Is 7 not 8!
```

Casting

From object to float results in a notice
From integer to float can result in loss of precision

Casting

From boolean to string
- false is ""
- true is "1"
From float or integer to string
- Textual representation to string form with E notation
From object to string
- Becomes "Object"
From array to string
- Becomes Array
From NULL to string
- Becomes ""

Casting

From string to number
- If it does not have ., e, or E in it becomes integer otherwise float
- Valid part of string used then rest discarded
- Nothing valid means 0

String conversion to numbers from php ⁶

<?php
$foo = 1 + "10.5";                // $foo is float (11.5)
$foo = 1 + "-1.3e3";              // $foo is float (-1299)
$foo = 1 + "bob-1.3e3";           // $foo is integer (1)
$foo = 1 + "bob3";                // $foo is integer (1)
$foo = 1 + "10 Small Pigs";       // $foo is integer (11)
$foo = 4 + "10.2 Little Piggies"; // $foo is float (14.2)
$foo = "10.0 pigs " + 1;          // $foo is float (11)
$foo = "10.0 pigs " + 1.0;        // $foo is float (11)

Exceptions are sometimes made for complex data types.↩
SimpleXML objects from empty tags are also false.↩
http://www.php.net ↩
http://www.php.net ↩
http://www.php.net ↩
http://www.php.net ↩

Intro to PHP 2

About PHP

Variables

Variables

Variable Primitives

Scalars

Boolean

Integer

Integer

Integer

Integer Overflow

Float

float aka double

Float

Float

NaN

String

String

String

String

String

String

String

String

String

String

Warning

Compound

Special

Resource

NULL

Type Juggling

Type Juggling

Casting

Casting

Casting

Casting

Casting

Casting

`float` aka `double`