PHP error category
Headers & Output
HTTP output ordering.
Headers & Output errors arise when http output ordering affect an executing PHP application or its supporting service. Use the exact signature and runtime context to narrow the cause before changing configuration.
The guides in this category separate code defects from environment differences, show focused diagnostics, and favor site-scoped corrections that preserve unrelated applications.
Guides in this category
| Title | Pattern | Severity |
|---|---|---|
| Cannot modify header information — headers already sent | Warning: Cannot modify header information - headers already sent by (output started at %s:%d) in %s on line %d |
warning |
| Cannot send session cache limiter or cookie | Warning: session_start(): Cannot send session %s - headers already sent |
warning |
| Output buffering does not fix header ordering | Warning: Cannot modify header information - headers already sent by (output started at %s:%d) |
warning |
| session_start cannot run after output | Warning: session_start(): Session cannot be started after headers have already been sent |
warning |