Description:
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mysqlnd allocates result objects in mysqlnd_result.c:1899 (mysqlnd_result_init):
	MYSQLND_RES * ret = mnd_pecalloc(1, alloc_size, persistent);

Note that these objects may be allocated persistently if the underlying connection is persistent. Normally these objects will be free'd in mysqlnd_result.c:347 (mysqlnd_res::free_result_internal):
	mnd_pefree(result, result->persistent);
Here, the persistency flag is respected.

However, in some cases (query errors or connection loss), mysqlnd_ps.c will use mnd_efree() instead of mnd_pefree() to free a result object:
line 132 (mysqlnd_stmt::store_result):
	mnd_efree(stmt->result);
line 359 (mysqlnd_stmt_prepare_read_eof):
	mnd_efree(stmt->result);

This will cause segfaults/"zend_mm_heap corrupted" in the above-mentioned cases.

Test script:
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<?php

$conn = new PDO("mysql:...", "...", "...", [PDO::ATTR_PERSISTENT => true, PDO::ATTR_EMULATE_PREPARES => false]);
$conn->query("select (select 1 union select 2)");

Expected result:
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Script executes successfully.

Actual result:
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segfault