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Map, Walk

Apply a user function to every member of an array. 
 
/**
 * array_map, array_walk()
 * 
 * Array walk needs reference
 */

function cube($n) 
{
    return pow($n, 3);
}

$A = [1, 2, 3];
$B = [1, 2, 3];
$C = [1, 2, 3];
$D = [1, 2, 3];

$A = array_map('cube', $A); 
$B = array_map(fn($x) => pow($x, 3), $B);

array_walk($C, fn(&$x) => $x = pow($x, 3));
array_walk($D, fn(&$x) => $x *= 2); // Look Here

print_r($A); // 1 8 27
print_r($B); // 1 8 27
print_r($C); // 1 8 27
print_r($D); // 2 4 6

Next

We can use internal pointer when working with arrays. 
 
/**
 * Reset, key, next, current ...
 */

$arr = ['a'=>1, 'b'=>2, 'c'=>3];

reset($arr);
while (key($arr) !== NULL) {
    print_r( key($arr) . current($arr) . " " ); // a1 b2 c3
    next($arr);
}

Exists

Check to see if array exists or not. 
 
/**
 * Check if array exists
 * Use is_array() / not count()
 */

$a = array();

echo count($a); // 0
echo is_array($a); // true

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