STEP 1 — DEFINE EXPECTED BEHAVIOR 🧭 Expected lifecycle 1. WooCommerce prepares the cart item price. 2. woocommerce_cart_item_price is applied. 3. The custom callback custom_cart_price() receives: $price $cart_item $cart_item_key 4. The callback detects custom_discount. 5. It creates the new formatted price. 6. The callback returns that value. 7. WooCommerce continues processing the filtered value. 8. The cart displays the modified price. Expected result Original price: ₹699 custom_discount: ₹499 Expected displayed price: ₹499 Important assumption The supplied code shows that $price is modified locally, but it does not show that the modified value is returned. That is a critical observation. --- STEP 2 — IDENTIFY HOOK TYPE 🪝 woocommerce_cart_item_price is a WooCommerce filter. Classification Hook type: Filter Component: WooCommerce Context: Frontend cart Purpose: Modify the cart item's displayed price Dynamic hook: No evidence that it is dynamic in this example Custom hook: No WordPress core hook: No Plugin hook: Yes Because this is a filter, the callback must return the value that should continue through the filter chain. --- STEP 3 — ANALYZE HOOK REGISTRATION 📝 Registration: add_filter('woocommerce_cart_item_price', 'custom_cart_price', 10, 3); This means: Hook: woocommerce_cart_item_price Callback: custom_cart_price Priority: 10 Accepted arguments: 3 The callback is therefore expected to be compatible with: custom_cart_price($price, $cart_item, $cart_item_key) The supplied signature matches the declared accepted-argument count. Registration assessment Strongly supported: The registration itself appears syntactically correct. However, registration correctness does not prove that the callback's output is correct. --- STEP 4 — ANALYZE EXECUTION TIMING ⏱️ The evidence says: custom_cart_price() executed custom_discount = 499 Therefore, there is direct evidence that the callback was reached during the reported cart request. This makes several hypotheses less likely: Callback never registered Callback registered too late Wrong request context Hook never executed Timing assessment Confirmed: The callback executes during the observed request. The remaining question is what happens to its output after execution. --- STEP 5 — CALLBACK ANALYSIS 🔎 The callback is: function custom_cart_price($price, $cart_item, $cart_item_key) { if (!empty($cart_item['custom_discount'])) { $price = wc_price($cart_item['custom_discount']); } } The callback receives the expected three parameters. The important issue is here: $price = wc_price($cart_item['custom_discount']); The function changes the local $price variable. But after that: return $price; is missing. Therefore, the callback does not return the transformed value. Filter behavior For a filter, the normal data flow is: Input ↓ Callback ↓ Modified value ↓ return modified value ↓ Next callback ↓ Final value Here, the flow becomes: ₹699 ↓ custom_cart_price() ↓ $price becomes ₹499 internally ↓ NO RETURN ↓ Filter chain receives no intended transformed value Callback assessment Confirmed issue: The callback does not return the filtered value. --- STEP 6 — PRIORITY ANALYSIS ⚖️ Custom callback: Priority: 10 Another plugin: Priority: 20 WordPress executes lower priority numbers first. Therefore: Priority 10 ↓ custom_cart_price() ↓ Priority 20 ↓ Other plugin callback So the callback at priority 10 runs before the callback at priority 20. Important distinction The fact that another plugin runs at priority 20 does not automatically mean priority 10 is wrong. The current evidence already identifies a more direct problem: the filter callback does not return its value. Priority assessment Not currently the root cause. Changing: 10 to: 20 or: 99 would not fix the missing return statement. --- STEP 7 — ACCEPTED ARGUMENT ANALYSIS 📦 Registration: add_filter('woocommerce_cart_item_price', 'custom_cart_price', 10, 3); Callback: function custom_cart_price($price, $cart_item, $cart_item_key) The counts are compatible: Accepted arguments: 3 Callback parameters: 3 The available evidence also indicates that: $cart_item['custom_discount'] contains 499. Assessment No accepted-argument mismatch is demonstrated. There is also no evidence of: missing callback arguments incorrect parameter count incorrect parameter order PHP argument errors --- STEP 8 — CALLBACK EXECUTION ORDER 🔄 The known chain is approximately: WooCommerce ↓ woocommerce_cart_item_price ↓ Priority 10 custom_cart_price() ↓ Priority 20 Other plugin callback ↓ Final displayed value However, the exact behavior of the second plugin cannot be established because its callback code was not provided. Therefore, we should not claim that the second plugin overwrites the value. Evidence distinction Confirmed: Custom callback executes. Custom callback runs at priority 10. Another callback is registered at priority 20. Unknown: What the priority-20 callback does. Whether it changes the price. Whether it expects the previous callback's return value. --- STEP 9 — FILTER DATA-FLOW ANALYSIS 🔀 Let's trace the value. Initial value $price = ₹699 Custom callback if (!empty($cart_item['custom_discount'])) { $price = wc_price(499); } Internally: ₹699 ↓ ₹499 But the callback ends without: return $price; Therefore, the intended transformed value is not explicitly passed back into the filter chain. Correct data flow ₹699 ↓ custom_cart_price() ↓ ₹499 ↓ return ₹499 ↓ next callback ↓ final WooCommerce output Current data flow ₹699 ↓ custom_cart_price() ↓ internal $price = ₹499 ↓ missing return ↓ broken filter chain behavior Root technical finding Confirmed: The callback fails to return the filtered value. --- STEP 10 — ACTION SIDE-EFFECT ANALYSIS ⚙️ This is a filter, not an action. Therefore, action-specific side effects such as: database writes emails API requests order creation are not the primary issue here. The important concern is value transformation and return behavior. Assessment Not applicable to the reported failure. --- STEP 11 — MISSING HOOK INVESTIGATION 🚫 The hook is not actually missing. Evidence: custom_cart_price() executed Therefore: Hook registration → likely successful Hook execution → confirmed through provided log This eliminates several potential causes. Unsupported hypotheses Wrong hook name: Unsupported Hook never fires: Contradicted by evidence Callback never registered: Contradicted by evidence Registration too late: Weakly supported at best --- STEP 12 — DUPLICATE EXECUTION INVESTIGATION 🔁 There is currently no evidence that: custom_cart_price() runs multiple times. The evidence only says: custom_cart_price() executed Therefore, duplicate execution cannot be confirmed. Verification Add temporary logging: error_log( 'custom_cart_price executed: ' . wp_json_encode([ 'count' => did_action('woocommerce_cart_item_price'), 'cart_item_key' => $cart_item_key, ]) ); However, did_action() is not the best standalone method for counting filter callback invocations. A dedicated temporary counter or backtrace-based logging can provide clearer evidence. Assessment Unknown. No duplicate-execution conclusion should be made from the current evidence. --- STEP 13 — RECURSION ANALYSIS ♻️ The callback only performs: wc_price(...) There is no supplied evidence that it: applies woocommerce_cart_item_price modifies the cart updates product metadata triggers another cart price calculation Therefore, recursion is not currently demonstrated. Assessment Unsupported by current evidence. No recursion protection should be added unnecessarily. --- STEP 14 — PLUGIN & THEME INTERACTION 🔌 Another plugin is known to use the same filter at priority 20. This is relevant because filters form a chain. Potential flow: WooCommerce price ↓ Custom callback @ 10 ↓ Other plugin @ 20 ↓ Final price But the other plugin's behavior is unknown. Therefore, it should not be blamed. Required verification Inspect: has_filter('woocommerce_cart_item_price'); And, where appropriate, inspect the registered callbacks and their priorities. The goal is to determine: Who is attached? At what priority? Which callback changes the value? What value enters each callback? What value leaves each callback? Assessment Potential contributing factor, not confirmed root cause. --- STEP 15 — DYNAMIC HOOK ANALYSIS 🧩 The supplied hook is: woocommerce_cart_item_price No dynamic construction such as: "woocommerce_{$something}" is shown. Therefore, dynamic-hook mismatch is not relevant to this case. Assessment Not applicable. --- STEP 16 — DEBUGGING & VERIFICATION 🛠️ The first verification should confirm the filter value entering and leaving the callback. A safe temporary diagnostic version: add_filter('woocommerce_cart_item_price', 'custom_cart_price', 10, 3); function custom_cart_price($price, $cart_item, $cart_item_key) { error_log('BEFORE: ' . $price); if (!empty($cart_item['custom_discount'])) { $price = wc_price($cart_item['custom_discount']); } error_log('AFTER: ' . $price); return $price; } Expected log: BEFORE: ₹699 AFTER: ₹499 Then inspect what happens in the next callback. If the second plugin's callback can be temporarily instrumented in a staging environment, log: Value received Value returned This reconstructs the actual filter chain. --- STEP 17 — ROOT CAUSE ANALYSIS 🎯 Hypothesis Evidence Assessment Wrong hook name Unsupported Wrong hook type Unsupported Callback not registered Contradicted Hook does not execute Contradicted Registration too late Weak Incorrect priority Unsupported as primary cause Accepted-argument mismatch Unsupported Callback signature mismatch Unsupported Missing filter return Confirmed Another plugin overwrites value Plausible but unverified Duplicate registration Unknown Recursion Unsupported Conditional registration failure Contradicted by execution evidence Strongest diagnosis The supplied callback changes $price but does not return it. Because woocommerce_cart_item_price is a filter, the transformed value must be returned so the filter chain can continue with that value. --- STEP 18 — FIX STRATEGY 🔧 The smallest correction is: add_filter('woocommerce_cart_item_price', 'custom_cart_price', 10, 3); function custom_cart_price($price, $cart_item, $cart_item_key) { if (!empty($cart_item['custom_discount'])) { $price = wc_price($cart_item['custom_discount']); } return $price; } What changed? Only this: return $price; was added. Why this is the smallest reliable correction It does not unnecessarily change: hook name priority WooCommerce core plugin files theme files accepted arguments callback signature The correction directly addresses the demonstrated filter-data-flow problem. --- STEP 19 — FIX VALIDATION ✅ Test the original failure condition. Test 1: Discount exists Original price: ₹699 custom_discount: ₹499 Expected: ₹499 Test 2: Discount does not exist Original price: ₹699 custom_discount: absent Expected: ₹699 Test 3: Multiple cart items Item A → custom_discount = ₹499 Item B → no custom_discount Expected: Item A → ₹499 Item B → original price Test 4: Second plugin interaction Verify that the priority-20 callback still receives the expected value. Test 5: Refresh/cart update Verify: normal cart page quantity update AJAX cart update if applicable mini-cart if relevant different products --- STEP 20 — REGRESSION PREVENTION 🛡️ Add a focused test around the filter behavior. Conceptually: Given: A cart item has custom_discount = 499 When: woocommerce_cart_item_price is applied Then: The resulting price contains the expected formatted 499 value. Also test the no-discount path: Given: No custom_discount exists Then: The original price remains unchanged. If another plugin depends on this same filter, include an integration test that verifies the expected callback chain. --- 🚨 PRODUCTION SAFETY Before changing production: Test the corrected callback on staging first. Confirm the displayed price is correct for multiple products. Check cart quantity updates. Check AJAX cart behavior where applicable. Verify that the second plugin still behaves correctly. Avoid modifying WooCommerce plugin files. Keep the change limited to the custom callback. Monitor logs after deployment. Have the previous PHP snippet available for rollback. Because this is a display-price filter, also verify whether the business requirement concerns only the displayed cart price or the actual calculated cart/order price. Changing woocommerce_cart_item_price does not automatically mean the underlying order total has changed. --- 📋 FINAL ENGINEERING ANALYSIS Hook Problem Summary The WooCommerce cart price callback executes, but the expected modified price does not persist through the filter chain. Expected Execution Flow WooCommerce → woocommerce_cart_item_price → custom_cart_price @ priority 10 → other callback @ priority 20 → final displayed price Actual Behavior WooCommerce → custom_cart_price executes → $price becomes ₹499 locally → callback does not return $price → expected transformed value is not properly propagated Registration Appears correct. Execution Timing Callback execution is confirmed by the supplied log. Callback Confirmed problem: missing return value. Priority Priority 10 executes before priority 20. No evidence currently establishes priority as the root cause. Accepted Arguments 3 accepted arguments match the 3 callback parameters. Execution Order Priority 10 → custom callback Priority 20 → other plugin callback The exact behavior of the second callback remains unverified. Root Cause Confirmed: the filter callback does not return the modified $price. Secondary Possibility Plausible but unverified: the priority-20 callback may further modify the value. Smallest Fix return $price; Regression Focus Verify: Correct discounted display + Original display without discount + Multiple cart items + Cart updates + Interaction with the priority-20 callback Engineering Conclusion The available evidence points first to a filter return-value failure, not a hook-priority problem. The priority-20 plugin should only be investigated further if the corrected callback still produces an unexpected final value.