J'ai rajouté le chemin de mon certificat dans le php.ini ( sur ma debian: /etc/php5/apache2/php.ini )
Et ça a réglé le problème.
[openssl]
; The location of a Certificate Authority (CA) file on the local filesystem
; to use when verifying the identity of SSL/TLS peers. Most users should
; not specify a value for this directive as PHP will attempt to use the
; OS-managed cert stores in its absence. If specified, this value may still
; be overridden on a per-stream basis via the "cafile" SSL stream context
; option.
openssl.cafile=MON_CERTIFICAT