Why doesn't ldap filter work fine with "_" character?

Question

PHP

Hello !
In general, there is a php code that accesses Ldap and takes data from there.
But I, unfortunately, have something like this field
u_someinfo
, when inserted into the ldap filter, it generates the error "Bad search ...". I tried to insert
the same field, but without "_" no errors came out, although there is no such field in ldap.
What's the matter?
Thanks everyone!
Best regards,
Alik