php ini implicit_flush

Enabling implicit_flush results in automatically clearing, or flushing, the output buffer of its contents after each call to print() or echo(), and completing each embedded HTML block.

This might be useful in an instance where the server requires an unusually long period of time to compile results or perform certain calculations. In such cases, you can use this feature to output status updates to the user rather than just wait until the server completes the procedure.

implicit_flush = Off

Note:This directive is hardcoded to On for the CLI SAPI

Implicit flush tells PHP to tell the output layer to flush itself automatically after every output block. This is equivalent to calling the PHP function flush() after each and every call to print() or echo() and each and every HTML block. Turning this option on has serious performance implications and is generally recommended for debugging purposes only.