I wrote about using PositiveSSL with Nginx before.
After 4 years, for Haproxy, the steps are pretty much the same, except the last one.
From the issuing email, donwload the certificate file, unzip it, we got two files: example_com.crt and example_com.ca-bundle
For Haproxy, we need to concatenate these two file with the private key file, but the order of concatenation is different from that for Nginx:
cat example_com.crt example_com.ca-bundle example.com.key.nopass > example.com.crt
Then put example.com.crt in to the directory that`s specified by Haproxy configuration file and restart Haproxy.
(Update 2015-05) I got a new Positive ssl cert, but this time I got different files. The concatenation step now looks like:
cat example_com.crt COMODORSADomainValidationSecureServerCA.crt COMODORSAAddTrustCA.crt AddTrustExternalCARoot.crt example.com.key.nopass > example.com.crt