array_flip() function swaps the key and values of an associative array, the values become key of the array and the keys become their related values.

<?PHP
	$arr=array("John"=>"4125643421","David"=>"2159032134");
	$arr2=array_flip($arr);
	foreach($arr2 as $key=>$val) echo "$key, $val; ";
	//4125643421, John; 2159032134, David; 
?>

If there are two same values in the original associative array, then only the last value will be flipped in the new array.

<?PHP
	$arr=array("apple"=>"carbon","rice"=>"carbon","nuts"=>"fat","meat"=>"protein");
	$arr2=array_flip($arr);
	foreach($arr2 as $key=>$val) echo "$key, $val; ";
	//carbon, rice; fat, nuts; protein, meat; 
?>

Although apple and rice both are carbon, after flip, only rice is saved.