Here are the information about debug options on mysqli_debug()
O,o:MYSQLND_DEBUG_FLUSH
A,a:MYSQLND_DEBUG_APPEND
F:MYSQLND_DEBUG_DUMP_FILE
L:MYSQLND_DEBUG_DUMP_LINE
m:MYSQLND_DEBUG_TRACE_MEMORY_CALLS
n:MYSQLND_DEBUG_DUMP_LEVEL
o:output to file
T:MYSQLND_DEBUG_DUMP_TIME
t:MYSQLND_DEBUG_DUMP_TRACE
x:MYSQLND_DEBUG_PROFILE_CALLS
f:? still investigating
For example, mysqli_debug("T:n:t:m:x:F:L:o,/tmp/client.trace");
mysqlnd will write Time, Level, trace, memory calls, profile calls, File, Line to client.trace file.
22:35:42.704501 ../mysqlnd_connection.c: 269 0:>mysqlnd_connection_init
22:35:42.704538 ../mysqlnd_driver.c: 10 1:| >mysqlnd_driver::get_connection
22:35:42.704549 ../mysqlnd_driver.c: 10 2:| | info : persistent=1
22:35:42.704558 ../mysqlnd_alloc.c: 21 2:| | >_mysqlnd_pecalloc
22:35:42.704570 ../mysqlnd_alloc.c: 23 2:| | <_mysqlnd_pecalloc (total=3 own=3 in_calls=0)
22:35:42.704602 ../mysqlnd_alloc.c: 21 2:| | >_mysqlnd_pecalloc
22:35:42.704626 ../mysqlnd_alloc.c: 23 2:| | <_mysqlnd_pecalloc (total=2 own=2 in_calls=0)
22:35:42.704650 ../mysqlnd_connection.c: 15 2:| | >mysqlnd_error_info_init
22:35:42.704675 ../mysqlnd_connection.c: 10 3:| | | >mysqlnd_error_info::reset
22:35:42.704697 ../mysqlnd_connection.c: 10 3:| | | <mysqlnd_error_info::reset (total=1 own=1 in_calls=0)
22:35:42.704725 ../mysqlnd_connection.c: 16 2:| | <mysqlnd_error_info_init (total=51 own=50 in_calls=1)
22:35:42.704744 ../mysqlnd_connection.c: 21 2:| | >mysqlnd_connection_state_init
If there is anything wrong, please let me know.