Description:
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From the documentation: 

ldap_modify() function is used to modify the existing entries in the
LDAP directory. The structure of the entry is same as in ldap_add().

...and 

bool ldap_add ( resource link_identifier, string dn, array entry)

The replace also has the same prototype:
bool ldap_mod_replace ( resource link_identifier, string dn, array entry)

Where, according to the doc, 'resource link_identifier' is the connection, 'string dn' is who to bind as and 'array entry' is the modified data.  No ability to specify *what* to modify so we will have to perform a separate action to retrieve this.  In the meantime the data has been changed by another process, perhaps.

Allow me to further illustrate:

If Jon's email address is the old one, update it to the new one.  This activity cannot be safely performed because using PHP you have to implement it in a two step process creating a potential race condition.