There are two types of comparison (opens new window): loose comparison with == and strict comparison with ===. Strict comparison ensures both the type and value of both sides of the operator are the same.

// Loose comparisons
var_dump(1 == 1); // true
var_dump(1 == "1"); // true
var_dump(1 == true); // true
var_dump(0 == false); // true

// Strict comparisons
var_dump(1 === 1); // true
var_dump(1 === "1"); // false
var_dump(1 === true); // false
var_dump(0 === false); // false

// Notable exception: NAN — it never is equal to anything
var_dump(NAN == NAN); // false
var_dump(NAN === NAN); // false

You can also use strong comparison to check if type and value don't match using !==.

A typical example where the == operator is not enough, are functions that can return different types, like strpos (opens new window), which returns false if the searchword is not found, and the match position (int) otherwise:

if(strpos('text', 'searchword') == false)
  // strpos returns false, so == comparison works as expected here, BUT:
if(strpos('text bla', 'text') == false)
  // strpos returns 0 (found match at position 0) and 0==false is true.
  // This is probably not what you expect!
if(strpos('text','text') === false)
  // strpos returns 0, and 0===false is false, so this works as expected.

Boolean (opens new window) is a type, having two values, denoted as true or false.

This code sets the value of $foo as true and $bar as false:

$foo = true;
$bar = false;

true and false are not case sensitive, so TRUE and FALSE can be used as well, even FaLsE is possible. Using lower case is most common and recommended in most code style guides, e.g. PSR-2 (opens new window).

Booleans can be used in if statements like this:

if ($foo) { //same as evaluating if($foo == true)
    echo "true";
}

Due to the fact that PHP is weakly typed, if $foo above is other than true or false, it's automatically coerced to a boolean value.
The following values result in false:

• a zero value: 0 (integer), 0.0 (float), or '0' (string)
• an empty string '' or array []
• null (the content of an unset variable, or assigned to a variable)

Any other value results in true.

To avoid this loose comparison, you can enforce strong comparison using ===, which compares value and type. See Type Comparison (opens new window) for details.

To convert a type into boolean, you can use the (bool) or (boolean) cast before the type.

var_dump((bool) "1"); //evaluates to true

or call the boolval (opens new window) function:

var_dump( boolval("1") ); //evaluates to true

Boolean conversion to a string (note that false yields an empty string):

var_dump( (string) true ); // string(1) "1"
var_dump( (string) false ); // string(0) ""

Boolean conversion to an integer:

var_dump( (int) true ); // int(1)
var_dump( (int) false ); // int(0)

Note that the opposite is also possible:

var_dump((bool) "");        // bool(false)
var_dump((bool) 1);         // bool(true)

Also all non-zero will return 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)

$float = 0.123;

For historical reasons "double" is returned by gettype() (opens new window) in case of a float, and not simply "float"

Floats are floating point numbers, which allow more output precision than plain integers.

Floats and integers can be used together due to PHP's loose casting of variable types:

$sum = 3 + 0.14;

echo $sum; // 3.14

php does not show float as float number like other languages, for example:

$var = 1;
echo ((float) $var); //returns 1 not 1.0

Warning

Floating point precision

(From the PHP manual page (opens new window))

Floating point numbers have limited precision. Although it depends on the system, PHP typically give a maximum relative error due to rounding in the order of 1.11e-16. Non elementary arithmetic operations may give larger errors, and error propagation must be considered when several operations are compounded.

Additionally, rational numbers that are exactly representable as floating point numbers in base 10, like 0.1 or 0.7, do not have an exact representation as floating point numbers in base 2 (binary), which is used internally, no matter the size of the mantissa. Hence, they cannot be converted into their internal binary counterparts without a small loss of precision. 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....

So never trust floating number results to the last digit, and do not compare floating point numbers directly for equality. If higher precision is necessary, the arbitrary precision math functions and gmp functions are available.

A string in PHP is a series of single-byte characters (i.e. there is no native Unicode support) that can be specified in four ways:

Single Quoted

Displays things almost completely "as is". Variables and most escape sequences will not be interpreted. The exception is that to display a literal single quote, one can escape it with a back slash ', and to display a back slash, one can escape it with another backslash \

$my_string = 'Nothing is parsed, except an escap\'d apostrophe or backslash. $foo\n';
var_dump($my_string);

/*
string(68) "Nothing is parsed, except an escap'd apostrophe or backslash. $foo\n"
*/

Double Quoted

Unlike a single-quoted string, simple variable names and escape sequences (opens new window) in the strings will be evaluated. Curly braces (as in the last example) can be used to isolate complex variable names.

$variable1 = "Testing!";
$variable2 = [ "Testing?", [ "Failure", "Success" ] ];
$my_string = "Variables and escape characters are parsed:\n\n";
$my_string .= "$variable1\n\n$variable2[0]\n\n";
$my_string .= "There are limits: $variable2[1][0]";
$my_string .= "But we can get around them by wrapping the whole variable in braces: {$variable2[1][1]}";
var_dump($my_string);

/*
string(98) "Variables and escape characters are parsed:

Testing!

Testing?

There are limits: Array[0]"

But we can get around them by wrapping the whole variable in braces: Success

*/

Heredoc

In a heredoc string, variable names and escape sequences are parsed in a similar manner to double-quoted strings, though braces are not available for complex variable names. The start of the string is delimited by <<<identifier, and the end by identifier, where identifier is any valid PHP name. The ending identifier must appear on a line by itself. No whitespace is allowed before or after the identifier, although like any line in PHP, it must also be terminated by a semicolon.

$variable1 = "Including text blocks is easier";
$my_string = <<< EOF
Everything is parsed in the same fashion as a double-quoted string,
but there are advantages. $variable1; database queries and HTML output
can benefit from this formatting.
Once we hit a line containing nothing but the identifier, the string ends.
EOF;
var_dump($my_string);

/*
string(268) "Everything is parsed in the same fashion as a double-quoted string,
but there are advantages. Including text blocks is easier; database queries and HTML output
can benefit from this formatting.
Once we hit a line containing nothing but the identifier, the string ends."
*/

Nowdoc

A nowdoc string is like the single-quoted version of heredoc, although not even the most basic escape sequences are evaluated. The identifier at the beginning of the string is wrapped in single quotes.

$my_string = <<< 'EOF'
A similar syntax to heredoc but, similar to single quoted strings,
nothing is parsed (not even escaped apostrophes \' and backslashes \\.)
EOF;
var_dump($my_string);

/*
string(116) "A similar syntax to heredoc but, similar to single quoted strings,
nothing is parsed (not even escaped apostrophes \' and backslashes \\.)"
*/

Callables are anything which can be called as a callback. Things that can be termed a "callback" are as follows:

$obj = new MyClass();
 call_user_func([$obj, 'myCallbackMethod']);

Callbacks can be denoted by callable type hint (opens new window) as of PHP 5.4.

$callable = function () {
    return 'value';
};

function call_something(callable $fn) {
    call_user_func($fn);
}

call_something($callable);

PHP represents "no value" with the null (opens new window) keyword. It's somewhat similar to the null pointer in C-language and to the NULL value in SQL.

Setting the variable to null:

$nullvar = null; // directly

function doSomething() {} // this function does not return anything
$nullvar = doSomething(); // so the null is assigned to $nullvar

Checking if the variable was set to null:

if (is_null($nullvar)) { /* variable is null */ }

if ($nullvar === null) {  /* variable is null */ }

Null vs undefined variable

If the variable was not defined or was unset then any tests against the null will be successful but they will also generate a Notice: Undefined variable: nullvar:

$nullvar = null;
unset($nullvar);
if ($nullvar === null) {  /* true but also a Notice is printed */ }
if (is_null($nullvar)) {  /* true but also a Notice is printed */ }

Therefore undefined values must be checked with isset (opens new window):

if (!isset($nullvar)) {  /* variable is null or is not even defined */  }

PHP will generally correctly guess the data type you intend to use from the context it's used in, however sometimes it is useful to manually force a type. This can be accomplished by prefixing the declaration with the name of the required type in parenthesis:

$bool = true;
var_dump($bool); // bool(true)

$int = (int) true;
var_dump($int); // int(1)

$string = (string) true;
var_dump($string); // string(1) "1"
$string = (string) false;
var_dump($string); // string(0) ""

$float = (float) true;
var_dump($float); // float(1)

$array = ['x' => 'y'];
var_dump((object) $array); // object(stdClass)#1 (1) { ["x"]=> string(1) "y" } 

$object = new stdClass();
$object->x = 'y';
var_dump((array) $object); // array(1) { ["x"]=> string(1) "y" }

$string = "asdf";
var_dump((unset)$string); // NULL

But be carefull: not all type casts work as one might expect:

// below 3 statements hold for 32-bits systems (PHP_INT_MAX=2147483647)
// an integer value bigger than PHP_INT_MAX is automatically converted to float:
var_dump(       999888777666 ); // float(999888777666)
// forcing to (int) gives overflow:
var_dump((int)  999888777666 ); // int(-838602302)
// but in a string it just returns PHP_INT_MAX
var_dump((int) "999888777666"); // int(2147483647)

var_dump((bool) []);      // bool(false) (empty array)
var_dump((bool) [false]); // bool(true)  (non-empty array)

A resource (opens new window) is a special type of variable that references an external resource, such as a file, socket, stream, document, or connection.

$file = fopen('/etc/passwd', 'r');

echo gettype($file);
# Out: resource

echo $file;
# Out: Resource id #2

There are different (sub-)types of resource. You can check the resource type using get_resource_type() (opens new window):

$file = fopen('/etc/passwd', 'r');
echo get_resource_type($file);
#Out: stream

$sock = fsockopen('www.google.com', 80);
echo get_resource_type($sock);
#Out: stream

You can find a complete list of built-in resource types here (opens new window).

PHP is a weakly-typed language. It does not require explicit declaration of data types. The context in which the variable is used determines its data type; conversion is done automatically:

$a = "2";             // string 
$a = $a + 2;          // integer (4) 
$a = $a + 0.5;        // float (4.5)
$a = 1 + "2 oranges"; // integer (3)

Integers in PHP can be natively specified in base 2 (binary), base 8 (octal), base 10 (decimal), or base 16 (hexadecimal.)

$my_decimal = 42;
$my_binary = 0b101010;
$my_octal = 052;
$my_hexadecimal = 0x2a;

echo ($my_binary + $my_octal) / 2;
// Output is always in decimal: 42

Integers are 32 or 64 bits long, depending on the platform. The constant PHP_INT_SIZE holds integer size in bytes. PHP_INT_MAX and (since PHP 7.0) PHP_INT_MIN are also available.

printf("Integers are %d bits long" . PHP_EOL, PHP_INT_SIZE * 8);
printf("They go up to %d" . PHP_EOL, PHP_INT_MAX);

Integer values are automatically created as needed from floats, booleans, and strings. If an explicit typecast is needed, it can be done with the (int) or (integer) cast:

$my_numeric_string = "123";
var_dump($my_numeric_string);
// Output: string(3) "123"
$my_integer = (int)$my_numeric_string;
var_dump($my_integer);
// Output: int(123)

Integer overflow will be handled by conversion to a float:

$too_big_integer = PHP_INT_MAX + 7;
var_dump($too_big_integer);
// Output: float(9.2233720368548E+18)

There is no integer division operator in PHP, but it can be simulated using an implicit cast, which always 'rounds' by just discarding the float-part. As of PHP version 7, an integer division function was added.

$not_an_integer = 25 / 4;
var_dump($not_an_integer);
// Output: float(6.25)
var_dump((int) (25 / 4)); // (see note below)
// Output: int(6)
var_dump(intdiv(25 / 4)); // as of PHP7
// Output: int(6)

(Note that the extra parentheses around (25 / 4) are needed because the (int) cast has higher precedence than the division)