Description:
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I propose a modification to the PHP core adding the following features. (Not all inclusive, I'll try to include everything I think needs to be added).

I guess this could be called interception. The official name is AOP.  What it would enable you to do is intercept method calls immediately before and after they are executed. It would also enable you to filter the arguments sent to the methods.

We would need:
A way to enable this feature.
A way to add interceptions
A way to list interceptions
A way to remove interceptions
The ability to view interceptions in standard debug backtraces.

Test script:
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interception::attachBefore('className', 'methodName', 'classToExecute', 'methodToExecute');
interception::attachFilter('className', 'methodName', 'classToExecute', 'methodToExecute');
interception::attachAfter('className', 'methodName', 'classToExecute', 'methodToExecute');

interception::detachBefore('className', 'methodName', 'classToExecute', 'methodToExecute');
interception::detachFilter('className', 'methodName', 'classToExecute', 'methodToExecute');
interception::detachAfter('className', 'methodName', 'classToExecute', 'methodToExecute');

$list = interception::getAll();
$list = interception::getForClass('className', [methodName]);

interception::clearAll();

Expected result:
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When attaching an intercept before a method call, the methodToExecute will receive the exact same arguments as the intercepted methodName. Immediately after methodToExecute has completed, methodName will then be executed.

When attaching an intercept filter, the methodToExecute method will receive the exact same arguments as the intercepted methodName passed by reference (maybe). It can then modify the arguments if required.

When attaching an intercept after a method call, the methodToExecute will receive the exact same arguments as the intercepted methodName in addition to the return value of methodName.