[PHP] Enable Mbstring

Mbstring is such a popular PHP extension that it is used everywhere and most libraries don't work without it. It can be compiled directly with PHP.
The official Docker images of PHP include it.

Author: julien-boudry

src/php/install.sh

@@ -169,7 +169,7 @@ install_php() {
         VERSION_CONFIG="--with-pear"
     fi
 
-    ./configure --prefix="${PHP_INSTALL_DIR}" --with-config-file-path="$PHP_INI_DIR" --with-config-file-scan-dir="$CONF_DIR" --enable-option-checking=fatal --with-curl --with-libedit --with-openssl --with-zlib --with-password-argon2 --with-sodium=shared "$VERSION_CONFIG" EXTENSION_DIR="$PHP_EXT_DIR";
+    ./configure --prefix="${PHP_INSTALL_DIR}" --with-config-file-path="$PHP_INI_DIR" --with-config-file-scan-dir="$CONF_DIR" --enable-option-checking=fatal --with-curl --with-libedit --enable-mbstring --with-openssl --with-zlib --with-password-argon2 --with-sodium=shared "$VERSION_CONFIG" EXTENSION_DIR="$PHP_EXT_DIR";
 
     make -j "$(nproc)"
     find -type f -name '*.a' -delete