PHP 7.1 allows for void and null return types by preceding the type declaration with a ? -- (e.g. function canReturnNullorString(): ?string) However resource is not allowed as a return type:<?phpfunction fileOpen(string $fileName, string $mode): resource{    $handle = fopen($fileName, $mode);    if ($handle !== false)    {        return $handle;    }}$resourceHandle = fileOpen("myfile.txt", "r");?>Errors with:Fatal error: Uncaught TypeError: Return value of fileOpen() must be an instance of resource, resource returned.

Developers with a C background may expect pass by reference semantics for arrays. It may be surprising that  pass by value is used for arrays just like scalars. Objects are implicitly passed by reference.<?php# (1) Objects are always passed by reference and returned by referenceclass Obj {    public $x;}function obj_inc_x($obj) {    $obj->x++;    return $obj;}$obj = new Obj();$obj->x = 1;$obj2 = obj_inc_x($obj);obj_inc_x($obj2);print $obj->x . ', ' . $obj2->x . "\n";# (2) Scalars are not passed by reference or returned as suchfunction scalar_inc_x($x) {    $x++;    return $x;}$x = 1;$x2 = scalar_inc_x($x);scalar_inc_x($x2);print $x . ', ' . $x2 . "\n";# (3) You have to force pass by reference and return by reference on scalarsfunction &scalar_ref_inc_x(&$x) {    $x++;    return $x;}$x = 1;$x2 =& scalar_ref_inc_x($x);    # Need reference here as well as the function sigscalar_ref_inc_x($x2);print $x . ', ' . $x2 . "\n";# (4) Arrays use pass by value sematics just like scalarsfunction array_inc_x($array) {    $array{'x'}++;    return $array;}$array = array();$array['x'] = 1;$array2 = array_inc_x($array);array_inc_x($array2);print $array['x'] . ', ' . $array2['x'] . "\n";# (5) You have to force pass by reference and return by reference on arraysfunction &array_ref_inc_x(&$array) {    $array{'x'}++;    return $array;}$array = array();$array['x'] = 1;$array2 =& array_ref_inc_x($array); # Need reference here as well as the function sigarray_ref_inc_x($array2);print $array['x'] . ', ' . $array2['x'] . "\n";

Be careful about using "do this thing or die()" logic in your return lines.  It doesn't work as you'd expect:<?phpfunction myfunc1() {    return('thingy' or die('otherthingy'));}function myfunc2() {    return 'thingy' or die('otherthingy');}function myfunc3() {    return('thingy') or die('otherthingy');}function myfunc4() {    return 'thingy' or 'otherthingy';}function myfunc5() {    $x = 'thingy' or 'otherthingy'; return $x;}echo myfunc1(). "\n". myfunc2(). "\n". myfunc3(). "\n". myfunc4(). "\n". myfunc5(). "\n";?>Only myfunc5() returns 'thingy' - the rest return 1.

Functions which return references, may return a NULL value. This is inconsistent with the fact that function parameters passed by reference can't be passed as NULL (or in fact anything which isnt a variable).i.e.<?phpfunction &testRet(){    return NULL;}if (testRet() === NULL){    echo "NULL";}?>parses fine and echoes NULL