Functions

Question 1

Can other functions and class definitions appear inside of a function?

Answer 1

Yes.

Question 2

What are the rules for function names?

Answer 2

Same as other labels in PHP: starts with a letter or underscore, followed by any number of letters, numbers, or underscores.

Question 3

When must functions be defined before they are referenced?

Answer 3

1. When a function is defined in a conditional manner:
$a = true;
foo(); //will not work yet
if($a == true) {
    function foo() {
        print "foo";
    }
}
foo(); //now it works

2. When a function is defined inside of another function which has not yet been called.

function foo() 
{
  function bar() 
  {
    echo "I don't exist until foo() is called.\n";
  }
}

/* We can't call bar() yet
   since it doesn't exist. */

foo();

/* Now we can call bar(),
foo()’s processing has
made it accessible. */

bar();

Question 4

All functions and classes in PHP have what scope?

Answer 4

Global.

Question 5

If a function or class was defined inside of a function, can it be called outside of it?

Answer 5

Yes, and vice-versa.

Question 6

Are function names case sensitive?

Answer 6

No.

Question 7

Can you call recursive functions in PHP?

Answer 7

Yes, but avoid over 100-200 recursion levels, as it can cause a termination of the current script.

Question 8

By default, function arguments are passed by ____

Answer 8

Value.

Question 9

To allow a function to modify its arguments…

Answer 9

…the arguments must be passed by reference.

Question 10

To have an argument to a function ALWAYS be passed by reference…

Answer 10

…prepend an ampersand to the argument name in the function definition.

Question 11

What are the limits on a default value for a function?

Answer 11

It must be a constant expression, not (for example) a variable, a class member, or a function call.

Question 12

When using default arguments in a function, any defaults should be…

Answer 12

…on the right side of any non-default arguments.

Question 13

Can arguments to a function that are passed by reference have a default value?

Answer 13

Yes.

Question 14

How does PHP support variable-length argument list for functions?

Answer 14

With the func_num_args(), func_get_arg(), and func_get_args() functions for PHP 5.5 and earlier.

Question 15

What types can be returned from a function?

Answer 15

Any type.

Question 16

If ‘return’ is omitted from a function, what is returned?

Answer 16

NULL

Question 17

How do you return a reference from a function?

Answer 17

Use the ampersand reference operator in both the function declaration and when assigning the returned value to a variable:

function &returns_reference()
{
    return $someref;
}

$newref =& returns_reference();

Question 18

What are variable functions?

Answer 18

If a variable name has parentheses appended to it, PHP will look for a function with the same name as whatever the variable evaluates to, and will attempt to execute it:

$func = ‘foo’;
$func(); // This calls foo()

Question 19

Variable functions won’t work with…

Answer 19

…language constructs such as echo, print, unset(), isset(), empty(), include, require, and the like.

Question 20

Can object methods be called with the variable functions syntax?

Answer 20

Yes:

$foo = new Foo();
$funcname = "Variable";
$foo->$funcname();  // This calls $foo->Variable()

Question 21

What will show which functions are loaded into PHP?

Answer 21

A call to phpinfo() or get_loaded_extensions().

Question 22

What happens if the parameters given to a function are not what it expects?

Answer 22

The return value of the function is undefined (usually it will return NULL).

Question 23

What are closures?

Answer 23

Also known as anonymous functions, they allow the creation of functions which have no specified name. They are most useful as the value of callback parameters.

Question 24

Can closures be used as the value of variables?

Answer 24

Yes. PHP automatically converts such expressions into instances of the Closure internal class.

Question 25

How do you assign a closure to a variable?

Answer 25

With the same syntax as any other assignment, including the trailing semicolon.

Question 26

Can closures also inherit variables from the parent scope?

Answer 26

Yes. Any such variables must be passed to the ‘use’ language construct.

Question 27

What are the two applications of the ‘use’ keyword?

Answer 27

It applies to namespace constructs and to closure constructs.

Question 28

When the ‘use’ keyword is used in a closure, to inherit a variable, can that variable later be changed, outside of the closure?

Answer 28

No. An inherited variable’s value is from when the function is defined, not when called. You can change this behavior by inheriting by reference.

Question 29

Can closures accept regular arguments?

Answer 29

Yes:

// Closures can also accept regular arguments
$example = function ($arg) use ($message) {
    var_dump($arg . ' ' . $message);
};
$example("hello");

Question 30

What is the difference between inheriting variables from the parent scope and using global variables?

Answer 30

Global variables exist in the global scope, which is the same no matter what function is executing. The parent scope of a closure is the function in which the closure was declared (not necessarily the function it was called from).

Question 31

Is it possible to use func_num_args(), func_get_arg(), and func_get_args() from within a closure.

Answer 31

Yes.