array_combine() is used to creates a new array by using the key of one array as keys and using the value of another array as values. Whereas array_merge() merges one or more than one array such that the value of second array appended at the end of the first array.
<?php
// array combine example
$fname = array("Jack","Sam","Maira");
$age = array("25","27","32");
$c = array_combine($fname,$age);
print_r( $c );
// OUTPUT - Array ( [Jack] => 25 [Sam] => 27 [Maira] => 32 )
// array merge example
$a1 = array("red","green");
$a2 =array("blue","yellow");
$a3 = array_merge( $a1, $a2 );
print_r( $a3 );
// OUTPUT - Array ( [0] => red [1] => green [2] => blue [3] => yellow )
?>