预定义常量
PHP Manual

Purpose checking flags

X509_PURPOSE_SSL_CLIENT (integer)
X509_PURPOSE_SSL_SERVER (integer)
X509_PURPOSE_NS_SSL_SERVER (integer)
X509_PURPOSE_SMIME_SIGN (integer)
X509_PURPOSE_SMIME_ENCRYPT (integer)
X509_PURPOSE_CRL_SIGN (integer)
X509_PURPOSE_ANY (integer)

预定义常量
PHP Manual