diff --git a/Zend/tests/gh18756.phpt b/Zend/tests/gh18756.phpt
new file mode 100644
index 00000000000..6e112d90604
--- /dev/null
+++ b/Zend/tests/gh18756.phpt
@@ -0,0 +1,13 @@
+--TEST--
+Bug GH-18756: Zend MM may delete the main chunk
+--EXTENSIONS--
+zend_test
+--FILE--
+<?php
+
+zend_test_gh18756();
+
+?>
+==DONE==
+--EXPECT--
+==DONE==
diff --git a/Zend/zend_alloc.c b/Zend/zend_alloc.c
index c7fa39cc143..92873be7bfc 100644
--- a/Zend/zend_alloc.c
+++ b/Zend/zend_alloc.c
@@ -2192,7 +2192,7 @@ ZEND_API size_t zend_mm_gc(zend_mm_heap *heap)
 				i++;
 			}
 		}
-		if (chunk->free_pages == ZEND_MM_PAGES - ZEND_MM_FIRST_PAGE) {
+		if (chunk->free_pages == ZEND_MM_PAGES - ZEND_MM_FIRST_PAGE && chunk != heap->main_chunk) {
 			zend_mm_chunk *next_chunk = chunk->next;
 
 			zend_mm_delete_chunk(heap, chunk);
diff --git a/ext/odbc/php_odbc.c b/ext/odbc/php_odbc.c
index 2eb994cad3d..7bc3a8bc17d 100644
--- a/ext/odbc/php_odbc.c
+++ b/ext/odbc/php_odbc.c
@@ -1461,6 +1461,7 @@ static void php_odbc_fetch_hash(INTERNAL_FUNCTION_PARAMETERS, int result_type)
 				if (rc == SQL_ERROR) {
 					odbc_sql_error(result->conn_ptr, result->stmt, "SQLGetData");
 					efree(buf);
+					zval_ptr_dtor(return_value);
 					RETURN_FALSE;
 				}
 
diff --git a/ext/pdo_odbc/odbc_stmt.c b/ext/pdo_odbc/odbc_stmt.c
index cd7ee7f9ebe..1f5009c746b 100644
--- a/ext/pdo_odbc/odbc_stmt.c
+++ b/ext/pdo_odbc/odbc_stmt.c
@@ -103,6 +103,7 @@ static int pdo_odbc_ucs22utf8(int is_unicode, zval *result)
 		zend_string *str = zend_string_alloc(ret, 0);
 		ret = WideCharToMultiByte(CP_UTF8, 0, (LPCWSTR) Z_STRVAL_P(result), Z_STRLEN_P(result)/sizeof(WCHAR), ZSTR_VAL(str), ZSTR_LEN(str), NULL, NULL);
 		if (ret == 0) {
+			zend_string_efree(str);
 			return PDO_ODBC_CONV_FAIL;
 		}
 
diff --git a/ext/zend_test/test.c b/ext/zend_test/test.c
index e55e79f9307..14aad0d1a32 100644
--- a/ext/zend_test/test.c
+++ b/ext/zend_test/test.c
@@ -1613,3 +1613,13 @@ static PHP_FUNCTION(zend_test_compile_to_ast)
 
 	RETVAL_STR(result);
 }
+
+static PHP_FUNCTION(zend_test_gh18756)
+{
+	ZEND_PARSE_PARAMETERS_NONE();
+
+	zend_mm_heap *heap = zend_mm_startup();
+	zend_mm_gc(heap);
+	zend_mm_gc(heap);
+	zend_mm_shutdown(heap, true, false);
+}
diff --git a/ext/zend_test/test.stub.php b/ext/zend_test/test.stub.php
index b1debcae740..528ec71ae7a 100644
--- a/ext/zend_test/test.stub.php
+++ b/ext/zend_test/test.stub.php
@@ -334,6 +334,8 @@ function zend_test_override_libxml_global_state(): void {}
     function zend_test_log_err_debug(string $str): void {}
 
     function zend_test_compile_to_ast(string $str): string {}
+
+    function zend_test_gh18756(): void {}
 }
 
 namespace ZendTestNS {
diff --git a/ext/zend_test/test_arginfo.h b/ext/zend_test/test_arginfo.h
index c49c032013e..07b519f3d6a 100644
--- a/ext/zend_test/test_arginfo.h
+++ b/ext/zend_test/test_arginfo.h
@@ -1,5 +1,5 @@
 /* This is a generated file, edit the .stub.php file instead.
- * Stub hash: 37ac76dddea2da24d3275cccc748b8fea4c8d09c */
+ * Stub hash: 13a5559e87cb073c921006bb3be5354b90247306 */
 
 ZEND_BEGIN_ARG_WITH_RETURN_TYPE_INFO_EX(arginfo_zend_test_array_return, 0, 0, IS_ARRAY, 0)
 ZEND_END_ARG_INFO()
@@ -177,6 +177,8 @@ ZEND_END_ARG_INFO()
 
 #define arginfo_zend_test_compile_to_ast arginfo_zend_create_unterminated_string
 
+#define arginfo_zend_test_gh18756 arginfo_zend_test_void_return
+
 #define arginfo_ZendTestNS2_namespaced_func arginfo_zend_test_is_pcre_bundled
 
 #define arginfo_ZendTestNS2_namespaced_deprecated_func arginfo_zend_test_void_return
@@ -315,6 +317,7 @@ static ZEND_FUNCTION(zend_test_cast_fread);
 static ZEND_FUNCTION(zend_test_is_zend_ptr);
 static ZEND_FUNCTION(zend_test_log_err_debug);
 static ZEND_FUNCTION(zend_test_compile_to_ast);
+static ZEND_FUNCTION(zend_test_gh18756);
 static ZEND_FUNCTION(ZendTestNS2_namespaced_func);
 static ZEND_FUNCTION(ZendTestNS2_namespaced_deprecated_func);
 static ZEND_FUNCTION(ZendTestNS2_ZendSubNS_namespaced_func);
@@ -440,6 +443,7 @@ static const zend_function_entry ext_functions[] = {
 	ZEND_FE(zend_test_is_zend_ptr, arginfo_zend_test_is_zend_ptr)
 	ZEND_FE(zend_test_log_err_debug, arginfo_zend_test_log_err_debug)
 	ZEND_FE(zend_test_compile_to_ast, arginfo_zend_test_compile_to_ast)
+	ZEND_FE(zend_test_gh18756, arginfo_zend_test_gh18756)
 #if (PHP_VERSION_ID >= 80400)
 	ZEND_RAW_FENTRY(ZEND_NS_NAME("ZendTestNS2", "namespaced_func"), zif_ZendTestNS2_namespaced_func, arginfo_ZendTestNS2_namespaced_func, 0, NULL, NULL)
 #else