I'm trying to understand array_merge() function

Question

I'm getting these results in that order ...

array(2) { [0]=> int(1) [1]=> int(2) }

WARNING array_merge(): Argument #2 is not an array on line number 12

NULL

WARNING array_merge(): Argument #1 is not an array on line number 14

NULL

And I'm trying to understand why ..

Here is my code :

$referenceTable = array();
$referenceTable['val1'] = array(1, 2);
$referenceTable['val2'] = 3;
$referenceTable['val3'] = array(4, 5);

$testArray = array();

$testArray = array_merge($testArray, $referenceTable['val1']);
var_dump($testArray);
$testArray = array_merge($testArray, $referenceTable['val2']);
var_dump($testArray);
$testArray = array_merge($testArray, $referenceTable['val3']);
var_dump($testArray);

Answer 1

The issue here is that, if either the first or second argument to array_merge() is not an array, the return value will be NULL

As a result, the call to $testArray = array_merge($testArray, $referenceTable['val2']) evaluates to $testArray = array_merge($testArray, 3) and, since 3 is not of type array , this call to array_merge() returns NULL , which in turn ends up setting $testArray equal to NULL . Then, when we get to the next call to array_merge() , $testArray is now NULL so array_merge() again returns NULL .

The fix for this is straightforward. If we simply typecast the second argument to an array, we will get the desired results. The corrected array_merge() calls would therefore be as follows:

$testArray = array_merge($testArray, (array)$referenceTable['val1']);
var_dump($testArray);
$testArray = array_merge($testArray, (array)$referenceTable['val2']);
var_dump($testArray);
$testArray = array_merge($testArray, (array)$referenceTable['val3']);
var_dump($testArray);

which will yield the following output:

array(2) { [0]=> int(1) [1]=> int(2) } 
array(3) { [0]=> int(1) [1]=> int(2) [2]=> int(3) } 
array(5) { [0]=> int(1) [1]=> int(2) [2]=> int(3) [3]=> int(4) [4]=> int(5) }