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Functions

A function is a piece of code which may be called using its name. It can take in arguments and return a value. Unlike variable names, function names are case-insensitive. Structuring a code with functions makes it readable and maintainable.
This function takes an array as an argument and returns an array.
<!DOCTYPE html>
<html>
<head></head>
<body><?php

function sum_product($arr){
   $s=0; $p=1;
   foreach ($arr as $v){
      $s+=$v;
      $p*=$v;
   }
   return [$s,$p];
}

$a=[10,20,30,40];
list($sum,$product)=sum_product($a);
echo $sum.",".$product;

?></body>
</html>
Default values, when passed to arguments after =, must appear at the end of the arguments list.

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<!DOCTYPE html>
<html>
<head></head>
<body><?php

function shout($s,$s1="I say: ",$s2="!"){
   echo $s1.$s.$s2."<br />";
}
shout ("Hello World");
shout ("Hello World","He says: ");
shout ("Hello World","She says: ",".");

?></body>
</html>
A string may be used as a function name.

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<!DOCTYPE html>
<html>
<head></head>
<body><?php
function shout(){
   echo "Hello World";
}
$f = "shout";
$f();
?></body>
</html>
A function needs not exist in a place before where it is called. However, if it is defined within an if-block or within a function, it needs to be 'executed' before it becomes visible globally.

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<!DOCTYPE html>
<html>
<head></head>
<body><?php
shout();
shout_again(); // error without first calling shout();

function shout(){
   echo "Hello World <br />";
   function shout_again(){
      echo "Hello World";
   }
}
?></body>
</html>
Variables inside a function are not visible outside the function. For global variables outside a function to be visible inside a function, they must first be referenced with global.

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<!DOCTYPE html>
<html>
<head></head>
<body><?php
$a=2; $b=3;
function add(){
   global $a, $b;//binds    
   $a=&$GLOBALS["a"];
   $b=&$GLOBALS["b"];
   $a+=$b;
   unset($a); // no effect on global $a, local $a unbound only
   $c=10;
}
add();
echo $a."<br />";
echo isset($c);  // false, NULL
?></body>
</html>
The value of a static variable in a function is preserved after the function ends. It resumes its value on the next function call.

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<!DOCTYPE html>
<html>
<head></head>
<body><?php
function f(){
    static $a=0;
    echo $a;
    $a++;
}
f();f();f();
?></body>
</html>
To allow a function to change the value of an external variable passed to it, use a reference symbol ‘&’ in the arguments list.

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<!DOCTYPE html>
<html>
<head></head>
<body><?php
function change_noref($n1,$n2){$n1++; $n2++;}
function change_ref(&$n1,&$n2){$n1++; $n2++;}
$a=5; $b=5;
change_noref($a,$b);echo ($a+$b)."<br />";
change_ref($a,$b);echo ($a+$b);
?></body>
</html>
To return a reference, include '&' before the function name, and include it again when assigning the function.

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<!DOCTYPE html>
<html>
<head></head>
<body><?php
$a=5;
function &ref(){
   global $a;
   return $a;
}
$b =& ref();
$a=10;
echo $b;
?></body>
</html>
An anonymous function can be assigned to a variable, or passed in as an argument to another function. When you declare an anonymous function, remember to add the semicolon at the end of the statement.

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<!DOCTYPE html>
<html>
<head></head>
<body><?php
$f = function(){echo "Hello World";};
function f2($func){
   $g=$func;
   $g();
}
f2($f);
?></body>
</html>
Sometimes we need to use variables in the parent scope in an anonymous function in PHP (unlike a Javascript closure).

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<html>
<head></head>
<body><?php
function f($s){
   $s2="I say";
   $f2= function () use ($s, $s2){
      echo $s2." ".$s;
   };
   $f2();
}
f("Hello World.");
?></body>
</html>
A function can take in any number of additional arguments. These arguments can be retrieved with func_num_args(), func_get_arg(i), func_get_args(). 

<!DOCTYPE html>
<html>
<head></head>
<body><?php
function f($v1,$v2){
   for ($i=0; $i<func_num_args(); $i++){
      echo func_get_arg($i);
   }
   echo "<br />";
   echo  func_get_args()[0].func_get_args()[1].func_get_args()[2];
}
f(1,2,3,4,5,6,7);
?></body>
</html>
A function may enforce its arguments to be of particular types. An enforced type can be an array, a callable, a class, or an interface. 

<!DOCTYPE html>
<html>
<head></head>
<body><?php

function f(array $v1, callable $fa, MyClass $ca){
   echo $fa().$ca->v2.$v1[1];
}

$g = function(){echo "Hello World";};
class MyClass{public $v2=100;}
$c = new MyClass;

f([100,200],$g,$c);

?></body>
</html>