Description:
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Spaces are transformed into underscores.

This might first of all cause collisions.. But the biggest problem is that the replacement is done on a reference of the variable, while it should be a copy.

Reproduce code:
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<?php

$q = "select field from table";

print   "key:$q\n";
memcache_get($server,$q);
print   "key:$q\n";

?>

Expected result:
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key:select field from table
key:select field from table

Actual result:
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key:select field from table
key:select_field_from_table