This happened to me also with PHP 5.4.1
Copying the offending DLL everywhere didn't worket, and I don't have Postgres installed in the server, but also planned to use PHP against different Postgres versions, so the only solution I found that worked was to put in httpd.conf a line like this:

LoadFile "C:/Program Files/PostgreSQL/8.4/bin/libpq.dll" 

but refering to the libpq.dll that comes bundled with PHP, like this:

LoadFile "C:/php/libpq.dll" 

After that it worked fine to me.

(c) https://stackoverflow.com/users/1373641/dayron-armas-peña
from: https://stackoverflow.com/questions/551734/php-not-loading-php-pgsql-dll-on-windows/10439560#10439560

If you get a message like this: 
"Php Startup: Unable to load dynamic library 'C:\php\ext\php_pgsql.dll"

1. In the Configuration File alter the order of the libraries "php_exif.dll" and "php_mbstring.dll", in such a way that is called first the library "php_mbstring.dll"

2. Copy all the libraries in the folder "bin" of your installation of PostgreSQL and paste them into the root folder of Windows

3. Uncomment the line "extension=php_pgsql.dll"
4. Now you should see on the page generated by "phpinfo()" section of PostgreSQL

Unable to load dynamic library 'C:\\php\\ext\\php_pgsql.dll' - The specified module could not be found.

another solution:

copy libpq.dll/all files from zip (if you have php from installer)

add to httpd.conf:

#BEGIN PHP INSTALLER EDITS - REMOVE ONLY ON UNINSTALL
PHPIniDir "c:/PHP/"
Loadfile "C:\php\php5ts.dll"
Loadfile "C:\php\libpq.dll"
LoadModule php5_module "c:/PHP/php5apache2_2.dll"
#END PHP INSTALLER EDITS - REMOVE ONLY ON UNINSTALL
AddHandler application/x-httpd-php .php
AddHandler application/x-httpd-php-source .phps

(in php.ini default: extension_dir ="c:\PHP\ext" )

if it doesn't work add c:\php (php path) to your windows path (and restart)

tested on php5.2.8 + apache2.2.3

If you get this error on Windows:

Unable to load dynamic library 'C:/php/ext\\php_pgsql.dll' - The specified module could not be found.

Try downloading the PostgreSQL binaries-no-install archive from http://www.postgresql.org/ftp/binary/v8.3.5/win32/ (replace 8.3.5 with the latest stable version) and copy these DLLs to C:/WINDOWS/system32/ :

comerr32.dll
gssapi32.dll
k5sprt32.dll
krb5_32.dll
libeay32.dll
libiconv2.dll
libintl3.dll
libpq.dll
ssleay32.dll