php -m |grep mysql
PHP Warning:  PHP Startup: Invalid library (maybe not a PHP library) 'mysqlnd.so' in Unknown on line 0

mprotect(0x7f382958c000, 12288, PROT_READ) = 0
openat(AT_FDCWD, "/usr/xxx/php/lib/php/extensions/no-debug-non-zts-20210902/mysqlnd.so", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\320\371\0\0\0\0\0\0"..., 832) = 832
newfstatat(3, "", {st_mode=S_IFREG|0755, st_size=285784, ...}, AT_EMPTY_PATH) = 0
mmap(NULL, 253824, PROT_READ, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f3829a03000
mprotect(0x7f3829a12000, 163840, PROT_NONE) = 0
mmap(0x7f3829a12000, 114688, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0xf000) = 0x7f3829a12000
mmap(0x7f3829a2e000, 45056, PROT_READ, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x2b000) = 0x7f3829a2e000
mmap(0x7f3829a3a000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x36000) = 0x7f3829a3a000
mmap(0x7f3829a40000, 3968, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f3829a40000
close(3)                                = 0
mprotect(0x7f3829a3a000, 20480, PROT_READ) = 0
munmap(0x7f3829a03000, 253824)          = 0
write(2, "PHP Warning:  PHP Startup: Inval"..., 103PHP Warning:  PHP Startup: Invalid library (maybe not a PHP library) 'mysqlnd.so' in Unknown on line 0
) = 103

config9.m4
……
dnl If some extension uses mysqlnd it will get compiled in PHP core