Programming PHP

If a string key from an earlier array is repeated, the earlier value is replaced by the
later value:
$first = array('bill' => 'clinton', 'tony' => 'danza');
$second = array('bill' => 'gates', 'adam' => 'west');
$merged = array_merge($first, $second);
// $merged = array('bill' => 'gates', 'tony' => 'danza', 'adam' => 'west')
Calculating the Difference Between Two Arrays
The array_diff( ) function identifies values from one array that are not present in
others:
$diff = array_diff(array1, array2 [, array ... ]);
For example:
$a1 = array('bill', 'claire', 'elle', 'simon', 'judy');
$a2 = array('jack', 'claire', 'toni');
$a3 = array('elle', 'simon', 'garfunkel');
// find values of $a1 not in $a2 or $a3
$diff = array_diff($a1, $a2, $a3);
// $diff is array('bill', 'judy');
Values are compared using ===, so1and "1" are considered different. The keys of the
first array are preserved, so in $diff the key of 'bill' is 0 and the key of 'judy' is 4.
Filtering Elements from an Array
To identify a subset of an array based on its values, use the array_filter( ) function:
$filtered = array_filter(array, callback);
Each value of array is passed to the function named in callback. The returned array
contains only those elements of the original array for which the function returns a
true value. For example:
function is_odd ($element) {
return $element % 2;
}
$numbers = array(9, 23, 24, 27);
$odds = array_filter($numbers, 'is_odd');
// $odds is array(0 => 9, 1 => 23, 3 => 27)
As you see, the keys are preserved. This function is most useful with associative
arrays.
Using Arrays
Arrays crop up in almost every PHP program. In addition to their obvious use for
storing collections of values, they’re also used to implement various abstract data
types. In this section, we show how to use arrays to implement sets and stacks.
136 | Chapter 5: Arrays
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