Description:
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I found the problem in configure by looking at helpful comments for bug number 19856.

I, too, was wondering why my copy of php was lacking pg_escape_string() even though I had just installed the latest version of Postgres (7.4.6).

It turns out that configure, when compling the conftest program to check for PQescapeString(), did this:

configure:70469: checking for PQescapeString in -lpq
configure:70488: gcc -o conftest -g -O2  -L/usr/lib -ldb-3.1  -Wl,-rpath,/usr//l
ib/mysql -L/usr//lib/mysql -L/usr/local/pgsql/lib conftest.c -lpq  -lmysqlclient
 -ldb-3.1 -lgdbm -lresolv -lm -ldl -lnsl  1>&5

There was an old copy of postgres in /usr/lib and that extra -L/usr/lib was causing gcc to favor it. This would be okay if the PHP build favored that, too, since I'd see in the postgres section of phpinfo() that the wrong version had been used. But the real compile of PHP doesn't use -L/usr/lib and finds the correct postgres install, yet wrongly does not include pg_escape_string().

I resolved this for myself by adding a "#define HAVE_PQESCAPE 1" in main/php_config.h by hand.