I believe it was fine to use persistent connections in ColdFusion because of the very different way both engines connect to mysql.
ColdFusion is basically a java engine using mysql's JDBC type4 driver. JDBC provides builtin connection pooling. When you use persistent connection, JDBC is able to manage dead connections.
PHP mysql modules connectis to mysql server directly with no inbetween connection pooling ability.
JDBC pooling method allows a more reliability operation when using persistent connections.