Description:
------------
A simple mysql_connect() returned NULL for this particular configuration of PHP: 
--with-mysql=/usr/lib/mysql
--with-mysql-sock=/var/run/mysqld/mysqld.sock
--with-mysqli=/usr/bin/mysql_config
--with-pdo-mysql=/usr

When the script is ran with wrong login credentials, mysql_connect() returns false as documented. However, when everything checks out no errors are thrown and NULL is returned instead of a resource.

This is not documented behaviour as far as I know ;-)

Reproduce code:
---------------
<?php

error_reporting(E_STRICT);

$r = mysql_connect('localhost','user','pw');

var_export($r);

?>


Expected result:
----------------
"Resource #1" (or something similar)

Actual result:
--------------
"NULL"

A strace on the process returns the following:
...
socket(PF_FILE, SOCK_STREAM, 0)         = 3
fcntl64(3, F_SETFL, O_RDONLY)           = 0
fcntl64(3, F_GETFL)                     = 0x2 (flags O_RDWR)
fcntl64(3, F_GETFL)                     = 0x2 (flags O_RDWR)
fcntl64(3, F_SETFL, O_RDWR|O_NONBLOCK)  = 0
connect(3, {sa_family=AF_FILE, path="/var/run/mysqld/mysqld.sock"}, 110) = 0
fcntl64(3, F_SETFL, O_RDWR)             = 0
setsockopt(3, SOL_IP, IP_TOS, [8], 4)   = -1 EOPNOTSUPP (Operation not supported)
setsockopt(3, SOL_SOCKET, SO_KEEPALIVE, [1], 4) = 0
poll([{fd=3, events=POLLIN, revents=POLLIN}], 1, 60000) = 1
read(3, "9\0\0\0\n5.0.24a-log\0\331\1\0\0TOlT=vZ?\0,\242"..., 16384) = 61
brk(0x8747000)                          = 0x8747000
stat64("/usr/share/mysql/charsets/Index.xml", {st_mode=S_IFREG|0644, st_size=18221, ...}) = 0
open("/usr/share/mysql/charsets/Index.xml", O_RDONLY|O_LARGEFILE) = 4
read(4, "<?xml version=\'1.0\' encoding=\"ut"..., 18221) = 18221
close(4)                                = 0
write(3, ">\0\0\1\205\242\0\0\0\0\0@!\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 66) = 66
read(3, "\7\0\0\2\0\0\0\2\0\0\0", 16384) = 11
fstat64(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 0), ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7f00000
write(1, "NULL", 4NULL)                     = 4
close(0)                                = 0

An ldd on the php cli:
...
   libmysqlclient.so.15 => /usr/lib/mysql/libmysqlclient.so.15 (0xb78f6000)