PHP and Oracle oci_connect() fails sometimes

In system level, I can connect to database using the wallet name, but sometimes, got below error:

1) PHP Warning:  oci_connect(): ORA-12715: invalid character set specified
2) ORA-01017: invalid username/password; logon denied
3)  PHP Warning:  oci_connect(): OCIEnvNlsCreate() failed. There is something wrong with your system


 PHP and Oracle oci_connect() fails sometimes, I tried change Character Set, cannot fix it.

In PHP,
putenv("TNS_ADMIN=/opt/.wallet");
putenv("ORACLE_HOME=/usr/app/oracle/product/11.2.0.3");
putenv("LD_LIBRARY_PATH=/usr/app/oracle/product/11.2.0.3/lib");

$conn = oci_connect("/", "", "WalletName", null, OCI_CRED_EXT);





In /etc/sysconfig/httpd:

export ORACLE_HOME=/usr/app/oracle/product/11.2.0.3
export LD_LIBRARY_PATH=/usr/app/oracle/product/11.2.0.3/lib
export TNS_ADMIN=/opt/.wallet