add ability to define abstract methods (#171)

* add ability to define abstract methods

`new_abstract` can be used for interface and abstract class methods.

* rustfmt

src/builders/class.rs

@@ -85,7 +85,7 @@ impl ClassBuilder {
     /// * `func` - The function entry to add to the class.
     /// * `flags` - Flags relating to the function. See [`MethodFlags`].
     pub fn method(mut self, mut func: FunctionEntry, flags: MethodFlags) -> Self {
-        func.flags = flags.bits();
+        func.flags |= flags.bits();
         self.methods.push(func);
         self
     }

src/builders/function.rs

@@ -1,7 +1,7 @@
 use crate::{
     args::{Arg, ArgInfo},
     error::{Error, Result},
-    flags::DataType,
+    flags::{DataType, MethodFlags},
     types::Zval,
     zend::{ExecuteData, FunctionEntry, ZendType},
 };
@@ -64,6 +64,30 @@ impl<'a> FunctionBuilder<'a> {
         }
     }
 
+    /// Create a new function builder for an abstract function that can be used
+    /// on an abstract class or an interface.
+    ///
+    /// # Parameters
+    ///
+    /// * `name` - The name of the function.
+    pub fn new_abstract<T: Into<String>>(name: T) -> Self {
+        Self {
+            name: name.into(),
+            function: FunctionEntry {
+                fname: ptr::null(),
+                handler: None,
+                arg_info: ptr::null(),
+                num_args: 0,
+                flags: MethodFlags::Abstract.bits(),
+            },
+            args: vec![],
+            n_req: None,
+            retval: None,
+            ret_as_ref: false,
+            ret_as_null: false,
+        }
+    }
+
     /// Creates a constructor builder, used to build the constructor
     /// for classes.
     ///